Fiscm R, with notes added Second English Edition, With 93 Illustrations, i8s net

LABORATORY MANUALOF

ELEMENTARY COLLOIDCHEMISTRYEMIL HATSCHEK

With 20

Illustrations

LONDON].7

&

A.

CHURCHILL1920

GREAT MARLBOROUGH STREET

Pnnt&d tn Great Britain

PREFACE.ALTHOUGH most of the existing text-books of Colloid Chemistry necessarily give, in more or less detail, descriptions of experimental piocedure and instructions for making many of the classical pieparations, no laboratoiy manual or collection of practical exeicises such as has been found indispensable in the teaching of other blanches of chemistry has so far appeared. The lack of such a work is all the more likely to check the spread of a practical knowledge of the discipline, as many of the methods and materials of colloid chemistry are peculiar, and strange even to students well trained in inorganic and organic chemi'stiy The present work is an attempt to 'fill this gap and to supply accurate and very detailed directions for carrying out the fundamental operations, for making a number of

representative preparations, and for examining them by the These are based throughout on personal standard methods described and of the difficulties experience of -the processes '--- ~- them""

expenenc'

The examples chosen

are,

simplest ones and, where alternainvolving the smallest expendituie and material The task of selection has not been easy, and the attempt to delimit the elementary legion the of the whole domain may seern premature or arbitrary guiding principle has been to provide for the wants of those students of numerous branches of science who are finding some training in colloid chemistry an indispensable part of their equipment, and are a151c to devote a limited time only to acquiring its technique For the guidance of readers desirous of going beyond the limits of this manual a number of refeiences to recent The papeis literature are given at the end of each section 1 J " 1 1 t- ircc, rl- of --,--' --expenme quoted arc *vv! io< iu- ^\ o to, or more which are o Ltr examples given in the text. Since the book is the first of its kind, the author will be very grateful for hints from readers who may find any of the directions given in it lacking in clearness or capable ofgenerally.

amount per volume CHAPTER VII EGG ALBUMIN SOL . Preparation and filtration of sol from dried albumin" Heat coagulation Coagulation by adsorption Salting out and the Hofmeister series of amons Reversal,.

Filtration Swelling and dispersion Effect of lyotropic series on setting and Sols containing a definite gelatin

.57

in acid sols Heavy metal precipitation tion of sol made from dried albumin and

white of eggs Crystallized albumin albumin sols to prevent dilution.

Purificafrom fresh Dialysis of

CONTENTS.CHAPTER VIIIclearingalkali

vii

PAGEEMULSIONS63Electiolyte coagulation and Concentrated emulsions Prepaiation with, Use of soap soluSeparation and phase-ratio tions and simple apparatus for same Phases of equal

THE LIESEGANG PHENOMENON

Original formula for silver chromate rings in gelatin Calcium phosphate in gelatin Lead iodide in agar Lead chromate in agar Molar concentration Indirect formation of rings Reactions in silicic acid gel. Pre-

124

servation of specimens.

NAME INDEXSUBJECT-MATTER INDEX.

x ^2

-133

A LABORATORY MANUAL OF ELEMENTARY COLLOID

CHEMISTRY.CHAPTERI

GENERAL REMARKS ON APPARATUS, MATERIALS AND PROCEDURE

in the operations to be with very few exceptions, that available in any chemical laboratory Glass vessels used for preparative work should, if possible, be of resistance this even test to tubes used for such glass applies work as experiments on electrolyte coagulation. Test tubes which turn distilled water containing a little phenolphthalem pink in a very short time are by no means uncommon, and should not be used As regards the choice of larger for any purpose. vessels, it should be remembered that very thorough

THE apparatus employed

is,

described

cleaning

is

necessary,

and thatis

in

many

cases

undue

undesirable, so that the choice will fall on tall cylindrical beakers, conical beakers with spout, or Erlenrneyer flasks. Flasks with narrow necks are, generally speaking, undesirable. Vessels should be cleaned immediately after use, in any event, and again before use in the case of The methods to be adopted sensitive preparations. in the former case naturally depend to a great degree on the previous contents of the vessel. Suspensoid sols are, ol course, easily washed off, although in

exposure of solutions to air

io

GENERAL REMARKS

some

cases especially with positive sols small quantities are adsorbed on the glass surface so that with dilute hydrochloric or tenaciously washing nitric acid may be required to remove the adsorbed

Very thorough and repeated washing is it must be continued necessary after emulsoid sols until the last water shows no tiace of froth Traces of gelatin, albumin, etc allowed to dry in glass vessels are veiy troublesome to remove, and may require the use of nitric acid or hot dichr ornateIrreversible gels, e g silicic sulphuric acid mixture should be made acid, formaldehyde-gelatin, etc in from vessels which the gel can be easily only removed, such as cylindrical beakers, preferably with thick walls Whatever the me.th.od of cleaning, the vessels should finally be rinsed thoroughly with distilled water and drained Drying with cloths is to be avoided drying with alcohol and ether is only necessary in the case of small apparatus used for viscometers. quantitative work, e g Apparatus cleaned as described and kept with the opening downwards will generally require only rinsing with several lots of distilled water before usefilm.,, ,,

Operations like making up salt solutions of known concentration for the preparation of sols, electrolyte coagulation, etc require the usual apparatus and call for no special remarks Sols of emulsoids cona taining definite amount of dry material to a given volume of dispersion medium also offer no difficulty Substances which disperse in the cold, like albumin or guni arabic, stick to the walls ol the vessel in the earlier stages of the process, but are easily detached when swelling has progressed sufficiently. The use of thick-walled vessels is, however, advisable, as thin beakers are easily broken in trying to detach If sols containfragments which stick obstinately ing a definite weight of substance in a given volume of sol are required, the procedure is a little more,

FILTRATION.

n,

difficult. For gelatin it is fully described under that with materials like albumin, gum, etc it heading will be found advisable not to make the sol directly The in the measuring flask, as stirring is impossible weighed quantity of dry material should be placed in a beaker and dispersed with, say, 50 or 60 per cent,

of the total

so obtained

volume eventually required, and the sol The poured into the measuring flask

beaker

is

then carefully rinsed with small successive

lots of dispersion medium, which are poured into the flask the aggregate volume of these washings mustfall

short of the

Thesion

flask is finally filled to the

medium from a

mixed

mark by a few cubic centimetres mark with the disperpipette and the contents well This method is not quite accurate, but the

error is generally not as great as that due to the variable moisture content of the starting material Filtration will be necessary chiefly in the case ofIt is generally a somewhat organic emulsoid sols tedious process and, whenever possible, should be left overnight The residues which have to be removed are generally not crystalline, and the use of vacuum does not accelerate the rate of filtration materially beyond the first few cubic centimetres Carefully folded filters may be used where only the filtrate is required, as the complete removal of residue Ribbed or corrugated from such filters is not easy glass funnels utilize the paper surface better than All smooth ones, but are not so easily cleaned funnels should have the spouts cut off quite short, say not more than 2 cm below the cone, as the usual long spouts are difficult to clean Fairly hard filter papers are advisable in most cases

filtered

Small quantities of troublesome sols may be through shredded asbestos with good results. This can be used in the ball tubes listed in most catalogues of chemical glassware in the following manner. A disc of silvei foil is cut which will pass

12

FILTRATION

easily through the upper part of the tube (Fig i), and this is perforated with a strong needle, being The disc supported on a cork plate for the purpose is then placed in the lower part of f the ball and pure shredded asbestos packed into it up to its junction with the tube. The asbestos must be introduced in small quantities, moistened with the solution to be filtered, and rammed down lightly with a stirring rod the exact degree of pressure,

required can be found only by experience The tube is then filled with the liquid to be filtered and the first few cubic centimetres of filtrate returned to it, if turbid. Concentrated sols like those of albumin or gumthis manner, will, arable, if filtered of course, generally be still opalesbut will be cent, sufficiently clear in

moderateeffect of

thicknesses to allow the coagulants to be seen dis-

tinctly,

and will be free from particles which would interfere with, say, vismeasurements cosity

centrifuge

capable

of

dealing

FIG

i.

with at least 100 c c. at a time will be found a very useful piece of apparatus, but is not indispensable. A microscope provided with a \" and a \ objective and with at least one high-power eyepiece, is required for the examination of sols by dark groundtt

The special features of these appliances are fully dealt with in the section devoted to them, but a general knowledge of the microscopeand ultra-condensersmust be presumed As regards materials, the most important oneis

DISTILLED

WATER

13

pure distilled water. If a sufficient amount of conductivity water is available, all difficulties are It is, however, not essential for the work avoided the following pages, and the extreme described precautions taken in the case of a few preparations mentioned the literature have proprominently duced a somewhat exaggerated impression of the

Water

standard of purity required for less delicate work. distilled with ordinary care reasonably designed apparatus will answer for all but a few purTrouble is much more likely to be caused poses through the ordinary storage vessels, since they are If trouble is very rarely made of resistance glass experienced, the first thing to do is to use freshly distilled water only, and to collect the small quantities which will be required in resistance glass flasks Storage vessels coated inside with paraffin wax may be used remember that the lining cannot be removed again by warming, as the glass cracks the wax before melts. For the few preparations which require exceptionally pure water small quantities may be redistilled a silver cooler. A thin-walled and condensed " silver tube about f bore is not expensive, and can a be fitted to easily Liebig condenser in place of the There must, of course, be no parts on glass cooler which water can condense, leading down the cooler in other words, the cooler must be bent down to the This may be done by filling the tube distilling flask. completely with fine, dry sand, corking both ends and bending slowly over a cylindrical object of 3" or The water should be redistilled from, and 4* radius

collected in, resistance glass flasks.

hardly necessary to add that throughout this distilled water of the standard available where redistilled water is essenDuality or tap -water permissible, this is specially tial, mentioned.It is

book water means

,

14

MATERIALS AND ACCURACY.

As regards the other materials, the ordinary The solutions made chemicals call for no remarks from them for such investigations as electrolyte coagulation are molar and not normal, and this should If there is any doubt about be borne in mind crystals containing the full amount of water of crystallization, or if the salts are anhydrous but A1C1 3 or d CNS, the solutions hygroscopic, e g must be standardized by the usual analytical,

NH

methodsMaterials lifce gelatin, agar or dried albumin are not definite chemical individuals, differ slightly or even considerably when obtained from different sources, and contain an amount of moisture which The best that can be hoped for varies perceptibly the first essential for this puris concordant results same use material throughout a given to the is pose investigation and, therefore, to start with a sufficient stock to allow for all contingencies These considerations have an obvious bearing on the degree of accuracy to be aimed at in weighing and measuring. Centi- or milli-molar solutions of electrolytes may be made up with the same care as solutions for volumetric analysis, although the operations in which they are eventually used have On the other hand, it is not a sharp end-point.,

unnecessary (and will, fortunately, also be found impossible with many atmospheric conditions) to weigh 10 gm of gelatin to fractions of a milligramme, since the moisture content may easily vary by 0-5 per cent of the total weight in a very short time. Nogeneral rules can be given which would be an adequate substitute for the exercise of common sense in thisrespect

A few general hints on procedure many

of

which

may

have not been found so by the author

this introduction.

appear superfluous to some reader or other, but -may conclude

VARIOUS HINTSRead the whole chapter before beginning any,

15of

the work described in it although the operations which they succeed are generally put in the order it is well to have a complete idea of the one another, work before starting.

Many preparations change with age do not make more than you require for immediate use or than will,

keep safely Label all preparations immediately in terms which, if not entirely correct technically, will remain This is particularly imporintelligible to yourself tant in the case of series, like solutions of different concentrations, Liesegang preparations, etc Adapt your methods to trie peculiarities of your For instance, when told to dilute a 5 per material cent collodion with an equal volume of acetic acid, do not put the highly viscous sol in the measuring vessel first and pour the thin solvent on it, but proceedin the reverse order

When an

experiment

fails,

repeat

it,

with the

a gold sol alteration of one factor at a time If, e,g turns out purple instead of red, try first a fresh carbonate and a so on. than fresh solution, beaker,

CHAPTER

II.

DIALYSIS.

THE cheapest and most convenient membrane for dialysmg any but small quantities say 50 to 100 c c. is parchment paper. This is readily obtainable in It varies a sheets or cut in squares of various sizes good deal permeability, and only an actual trial can decide whether a particular sample is satisfactory for a given purpose As the paper is fairly brittle in the dry state it should be kept flat or rolled, but never

folded.

The classical method of employing the parchment membrane is that used by Thomas Graham, whose

may be found in all catalogues of chemical It consists simply of a glass cylinder both ends, one of which is provided with a open rim or groove, over which the membrane is tied. A circular piece about 2" larger in diameter than the cylinder should be cut and thoroughly soaked in water, placed centrally over the rim, carefully turned down over it all round, and then tied with a thin Unless this is done with care, leakage may string take place through some of the folds formed before a possibly valuable solution is placed in the apparatus it should, therefore, be tested by filling it with water and ascertaining that it does ^ not escape (The same precaution applies to all dialysersdialyserat

apparatus

to be

described in this chapter

/)

The

pended or supported in a vessel filled with water, which is changed from time to time, or renewedcontinuously.

dialyser

is

sus-

PARCHMENTties,

BAGS.

17

Parchment bags are preferable

as a larger surfaceis

for larger quantiobtained in the same space

They may be made as follows Cut a regular hexagon and soak it thoroughly in water Then place it centrally on the bottom of an inverted beaker or ]ar,the diameter of which is about one-third of that of the inscribed circle of the hexagon Gently pinch radial folds from the circumference of the beaker to the corners of the hexagon and mould them so that the paper midway between the corners touches the wall of the beaker, and then turn the folded portions over and smooth them into cylindrical shape Thewill be quite clear from Fig 2, initial hexagon (dotted) and the the edge in plan, as well as a perspective view of the nearly completed bag on the beaker The folds must not be sharp, as even wet parchment may be damaged by too drastic treatment When the bag has been moulded as described, a string is loosely tied round it, or a fairly slack rubber band slipped over it within about 2" of the Its edge, and the bag is then drawn off the beaker permanent shape is secured by threading a clean, thin string through the folds, as indicated by dotted line in the plan, which is gently drawn tight after every completed stitch so that the circumference at the open end is approximately the same as at the bottom The bag is suspended in a jar of suitable size by two or three strings tied at equal distances to The ]ar the stung which secures the circumference is then slowly filled with water, while the liquid to be same time and dialysed is poured into the bag at the at about the same rate, so as to keep the external and in this way any internal level nearly the same strain on the mouth of the bag is avoided and it

whole procedure which shows the

final outline of

retains its shape. The water may be renewed from time to time, but it is preferable to use a continuous This flow, as dialysis is greatly accelerated thereby.

i8

FIG. 2

PARCHMENT TUBES.

19

may be done by allowing water to flow into the outer vessel and removing it by means of a syphon, which must be of the type shown in Fig. 3, to avoid either the vessel or the syphon being emptied, if the water The rate of supply supply fails by any accident must, of course, be so adjusted as not to exceed the rate of discharge from the syphon, since otherwisethe water may flow over the top of the ]ar. It is hardly necessary to add that the same arrangement

may be used

with a Graham dialyser, and also, with many of the appliances yet to be described. Continuous flow can, of course, be used only when the liquid remaining in the dialyserslightly modified,is all

that is wanted if it is, for any reason, necessary to examine the solution which has diffused tmough, dialysis must be performed with successive lots of water, which may be kept separate or be;

combined Parchment paper may also be obtained in the " " as they are form of tubes sausage-skin dialysers usually termed in catalogues They are sold flat, that condition are from 40 to 100 mm wide, and

giving a diameter,

when filled with liquid, of 25 to mm. A^they are easily damaged, any length selected for use should be carefully tested for leaks one of the simplest It may be used in various ways is to bend a (thoroughly soaked and tested) piece into U-shape and place it into a tall cylinder, allowThe ing the open ends to project an inch or two tube is then slowly filled with the liquid to be dialysed, while the cylinder is at the same time filled with water at about the same rate, so that no strain is placed on the tube. Another method is to close one end of the tube by folding it over two or three times, the first fold being about 5 mm, wide, and securing The clip is made by this end with a rubber clip70

wide and cutting a rectangular stnp about 20 about 25 mm. longer than the width of the (flat) tube

mm

2O

JFIG 3

PARCHMENT THIMBLES

21

from white rubber sheet about 8 to 10 thick, in which a central straight incision is made about 5 mm. This is then longer than the width of the tube opened a little by inserting two thin pieces of stick at the ends, slipped over the folded end, and then closed by withdrawing the sticks When the use of metal is unobjectionable one of the wire clips used for attaching papers to one another may be employed, or a similar clip bent from heavy silver or wire. The tube copper should be tested for leakage after being closed Finally, seamless thimbles of parchment papers can be obtained, which, although somewhat expensive, are reliable

mm

and

extremely convenient, especially for the examination of small quantities

of

liquid

A simple

method of using them is shown in Fig 4. The

thimble, filled with the solution to be dialysed, isFIG 4

placed in an Erlenmeyer flask of suitable size filled with the solvent The parchment swells perceptibly water, and the neck of the flask must, therefore, be a few millimetres larger in diameter than the (dry) thimble, to permit its easy withdrawal when it is saturated. A number of natural membranes, such as gold-

beater's skin, fish -bladder, etc have been used for Since they vary in permeability or require dialysis. careful purification, their use can hardly be recommended except as makeshifts, especially in view of,

the comparative ease with which membranes of considerable uniformity and covering a great range of

22

COLLODION THIMBLE

permeability can be made by the mett described These are based on the use of coll< a mixture of eth of cellulose nitrate The raw material is obtainable commei " " cotton or pyroxylin/ and is general it should be dried be with alcohol The usual concentration is 3 to 4 gm to 100 c c of ether- alcohol mixtuie, t of the lattei varying between 14 parts per cent ) to 86 of ether, and 25 parts equal volumes o 75 parts of ether

ether have also been used, but this

unusual

The weighed quantity of gun cotton wide-necked bottle, the requisite voh poured on it, the bottle corked and all for about fifteen minutes The ether is t the mixture stirred occasionally, until 1 so w it should do has dissolved The sol should be aln any residue One of 1 does not require nit ei ing,

vement ways of employing it is to n thimbles by coating the inside of test ti the beginner will find 20 size,

long a convenient size, practice much larger thimbles can be The test tubes must be qi difficulty the inside, thoroughly clean and dry125j

mm

mm

The

selected test tube is filled with

being taken to pour it down the side so any air bubbles. The mouth of the te placed above that of the bottle and

poured back slowly by slightly mclimn 1 rotating it constantly and slowly. of the tube is gradually increased asceeds,

but not more than

is

necessar

collodion to flow out in a thin uniform

COLLODION THIMBLES.is

23is

raised too rapidly the bottom of the thimble apt to be excessively thin. The tube is finally brought to a vertical position and the last remains of collodion,

which should not then amount to more than a few drops, allowed to drip off, after which the layer lefttime.

on the inside of the tube is allowed to dry for a short Although the degree of drying is the crucial point of the whole process, no definite rules can be given the collodion should not stick to the finger when touched lightly, and should just be visible as a faintly bluish coating when the tube is viewed When this stage is against a dark background reached the tube is submerged in water, care being taken to allow all air to escape, and is left for at least The depth of water should be about 15 minuses 2" more than the diameter of the tube, so that the subsequent operation can be carried out without its After the minimum time of being uncovered immersion has elapsed, the collodion film is detached round the edge of the tube, a finger inserted so as to touch the collodion skin, and the latter very slowly pulled out, while the test tube is held with the left hand. It must be remembered that ihe rate at which the collodion skin can be pulled out is fixed by the rate at which water can flow through the space between it and the wall of the test tube, which is necessarily slow any attempt to hurry matters is If the bottom of the vessel containing the fatal. water is dark, the collodion membrane can be seen very distinctly, and the bottom end, which is the most likely portion to give trouble, watched. The finished thimbles can be kept under water for several weeks, undergoing only slight changes in On the whole, however, it is advispermeability able to make and use them fresh They can be mounted in a variety of ways a convenient method is to insert a short piece of glass tubing, the edge of which has been carefully rounded in the flame, and, ;;

24

COLLODION THIMBLES

to fix the thimble to it with collodion, or by tying ; in the latter case a strip of gutta-percha tissue oroiled silk must be wound over the collodion to prevent it from being cut by the thread used for tying As has already been pointed out, the permeability made from ether-alcohol collodion of thimbles depends very largely on the extent of drying which The ether they have undergone before immersion and alcohol still remaining in the film is replaced by water, and this fixes the permeability of the hydrogel

which ultimately constitutes the Although practice soon enables a careworker to turn out fairly uniform thimbles, the whole difficulty can be avoided by the use pf acetic This is made by dissolving 4 gm of acid collodion. lower gun cotton in 100 c.c of glacial acetic acid concentrations give fragile films, while higher ones The test produce unnecessarily dense membranes tubes are coated and the excess emptied in exactly the film is, however, not the same way as described allowed to dry, but the coated tubes are immediately submerged in water. After about 30 minutes they may be withdrawn as explained, the operation being generally easier than with ether-alcohol collodion they are then left in water, which is occasionally changed, until the whole of the acetic acid has diffused out, and may be preserved under water, A very convenient way of making dialysmg thimbles, which are less fragile and permit much greater variations permeability than those just described, consists in impregnating the seamless thimbles of filter paper (Soxlilet thimbles) made in various sizes for fat extraction One of these is held vertically over a small dish and filled to the top with collodion when the liquid has penetrated over the entire surface, it is inverted and drained with conof cellulose nitrate

membrane

ful

stant turning.

If acetic

acid collodion

is

used, the

STAR DIALYSER.

25,

thimble is then submerged in water immediately with ether- alcohol collodion it must, like the thimbles formed in test tubes, be allowed to dry for a few minutes before immersion Since the mechanical strength is provided by the filter paper, collodions of low concentration may be employed, 2 per centin either acetic acid or ether-alcohol being sufficient for most purposes The thimbles are strong enough to stand upright, and may be used like parchment thimbles convenient method of using any paper type of thimble is to stand or suspend it in a cylindrical vessel of slightly larger diameter, provided with an inlet at the bottom and an overflow outlet at a level i to 2 cm below the top edge of the thimble. Water is continuously passed in at the bottom and overflows at the top, and dialysis proceeds with great rapidity with comparatively small quantities of water. Flat membranes of (ether- alcohol) collodion are rather easier to make than thimbles, and can con" Star veniently be used for continuous dialysis in the

" described by Zsigmondy. The apparatus Dialyser (Fig 5) consists of two parts, both of ebonite, a disc provided with a rim about 10 to 15 mm. deep, and a cylinder which fits loosely into the latter, open at both ends and 30 to 40 deep The disc has a central inlet and its upper face is provided with six or eight ribs, about 3 deep, which stop a few millimetres short of both the central opening and of the nm.

mm

mm

To prepare the membrane the ring is placed on a clean piece of plate glass and sufficient collodionpoured into it to cover the glass to a depth of 2 or The ring is lifted slightly, to allow the collodion to penetrate between it and the glass ; to strengthen the joint thus made, the outside of the ring is painted with collodion to a height of3

mm

about 5 mm. from its lower edge. After the collodion has dried some minutes water is poured into the ring, which, together with the collodion mem-

26

STAR DTALYSER.liftedoff

brane adhering to it, can be after about 10 or 15 minutes

the glassis

The

ring

then

FIG. 3

placed into the disc, filled with liquid to be dialysed, and water is passed through the central inlet, which

SPECIAL DIALYSERS

27

overflows round the edge of the rim. As it is difficult to adjust the apparatus so exactly that overflowis uniform all round the rim, it is best to localize it by means of two or three strips of filter paper, placed between

the open cylinder and the rim and bent over the latter, so as to act as syphons. Special arrangements are necessary when the sol to be dialysed has an appreciable osmotic pressure, as is the

In this the dialyser, diluting the sol and eventually causing it to overflow The only way to prevent this is to counterbalance the osmotic in other pressure hydrostatically words, to keep the level of the sol in the dialyser above the watei level outside from the beginning For smallcase, e g,

with albuminflowsinto

sols.

case water

quantities, such as come into question here, the simplest airangement is thator paper impregnated with collodion is fitted with a rubber stopper and tied tightly a strip of gutta-percha tissue about wide is first wound round the 15 end of the thimble and strong thread tied over this Thiough the rubber stopper passes a funnel tube about 30 cm long, which must have a diameter of at least 8 mm., so that the sol can be poured down one side of it, allowing the air to escape and the thimble and tube to be filled to the top. The a beaker thimble is submerged through which water flows con,

shown in Fig 6 A of either parchment

dialysing thimble^l'*

mm

tinuously.

With

this

arrangement

FIG. 6

28

DIALYSERS

the liquid remains at its original concentration and the bulk of it is still contained in the dialysing membrane, as the volume of the funnel tube is comparatively small

LITERATUREMembranes made from collodion of unusually high concentration and capable of standing considerable piessures are described by A. T Glenny and G. S. G Wegehn, Walpole, Eiochem fourn. IX. 284 (1915) Roll -Zeitschr XVIII, 225 (1916), a new method of and ultra-nltiation. rapid dialysis,,,

CHAPTER

III.

SUSPENSOID SOLSA.

METALLIC SOLSsolution of gold chloride

Gold Sols

i per cent

(more correctly, auro-chlorhydnc acid, HAuCl 4 " as the starting material The gold 3H 2 O) serves " of chloride photography, NaAuCl 4 2H 2 0, may be used instead for all the methods, with the exception a i per cent solution of this salt is of Zsigmondy's For the method first obtainable in commerce reduction described, by tannm, the gold chloride must be made exactly neutral to litmus by addition of sodium or potassium carbonate (N/5 solution) Reduction by tannin (Wo Ostwald) Dissolve o-i gm of purest tannin in 100 c c of water If this solutionns to be kept it should receive an addition of a few drops of chloroform, without which it goes mouldy. Dilute i c c. of the gold chloride solution with 200 c c. of water, stir and 'add i c c of the tannin Reducsolution, then warm over a Bunsen burner,

tion gradually proceeds Continue heating and,

and the liquid becomes red. when the liquid boils, add another cubic centimetre of gold chloride solution, followed by a cubic centimetre of tannm The resulting sol should be perfectly clear in transmitted -red colour of and The mixture deep ruby light must be well stirred after every addition Sometimes the colour of the liquid containing the first lot of gold chloride and tannm does not become le warming, but purple, or even a cold violet.

30

METALLIC SOLSnot be deterred, but continue to heat to boiling second addition of gold chloride and tannm changes to;

Do

after the

at boiling point the colour very generally red without even a tinge of purple

Reduction

may

also

be carried out in the cold by

using a larger proportion of tannin solution, say 100 c c of water, i c c of gold chloride solution, and In 3 to 5 c.c of tannin solution, added gradually. this case the sol is more liable to have a purple or bluish tinge Sols made by these methods are hable to the

growth of mould on keeping and gradually lose colour, the gold being deposited on the mycelium of This trouble may be prevented by the mould adding a few drops of chloroform, or, in view of the great simplicity of the method, by preparing the sols when and as required. These sols are protected to a slight and uncertain extent by the tannin and its oxidation products, and are less suitable for coagulation experiments than the following Heat Reduction by formaldehyde (R Zsigmondy). 120 to 150 c c of redistilled water in a 300-0 c while it is warming add I c c of gold beaker chloride solution (i per cent ), and then 2-5 to 3 c c As of a N/5 solution of purest potassium carbonate. add soon as the solution boils stir vigorously 11 " --Hr^-jt fairly quickly, 2 to 3 c c of dilute, ,

solution (i c c of commercial 40 per cent, formalin to 100 c c of water) and extinguish the flame. Reduction is complete in about a,<.,I

'

'

minute, and the resulting sol should be perfectly clear in transmitted light and of pure ruby-red colour without purple tinge. The beaker used should be of resistance glass, and stirring rods of the ordinary soft glass must not be used a tube of resistance glass closed at one end The sol should also be should be used for stirring;

kept in vessels of resistance

glass.

METALLICFor some reason, whichbatches than about 150 cc.

SOLS.isstill

31obscure, largersuccessfully

cannot,

be

made

If larger quantities are required,

they must be

made

since all the solutions 150 c.c lots as described can be made up in large quantities and" keep indefino is in there difficulty nitely, preparing any volume

be required. be dialysed against redistilled water, without this being done in vessels of but keep It is very suitable as a standard resistance glass. preparation for experiments on electrolyte coaguof sol likely to The sol can willlation, protection, etc

This can be prepared by exactly Palladium Sol the same procedure as Zsigmondy's gold sol, using the following quantities 150 c c of water, i c c of i per cent, palladium chloride solution, and 0-4 c c of N/io sodium carbonate solution, reduced by 3 to

c c

of dilute

formaldehydec.

(i c c

of

commercial

of water). The sol should be brown and perfectly clear in transmitted light Reduction by dextrine (Carey Lea's Silver Sol.

formalin to 100 c

method). This is one of the best examples of a highly concentrated metallic sol, as concentrations up to 5 per cent of Ag can be obtained in favourable The following quantities should be tried conditions first, but there is no difficulty in dealing with four or five times these amounts. 100 c.c. Dissolve 4 gm. of commercial dextrine of water and then 4 gm. of purest caustic soda. 20 c c. of water Dissolve 3 gm, of silver nitrate and add to the dextrine-soda solution. A precipioxide is which silver tate of forms, gradually reduced by the dextrine, the colour changing to a reddishbrown. Allow 20 to 30 minutes for this, and then add 100 c c of 96 per cent, alcohol and stir. Allow the mixture to settle for another 15 to 20 minutes, and then pour off the tuibid liquid from the sediment On pouring on of silver as completely as possible.

32

METALLIC SOLS.

water the silver generally disperses should this not be the case, a little slistlcJ stirring will be sufficient to induce dispersi* *n

The silver amounts to 1-81 gm and in f ;i conditions 35 to 40 c c of water will di^| < whole of it, so that the sol contains about f> } *,

It is, however, advisal >lt.' phase v < . a greater volume of water, say about 180 sol is dark brown and opaque even in thin when diluted with about 50 times the UJ*M transmitted ligltt water it should be clear * greenish-black surface colour in reflected lii 1 1 colour of the sol, and in fact the success of t I * method, depends a good deal on the qua lii

of disperse

dextrine,

which can be determined only 1*\"

speaking, the ordmarv commercial brands work better than a high I | r The per cent sol may be kept product for a long time Add to 100 c c. <> I Reduction by tannin ice of i per cent silver nitrate solution, tit a few drops of weak ammonia Reduce \v I \ The * -* *1 4 c c of 0-5 per cent tannin solution be brown and perfectly clear in transmit f *-< with a marked green surface colour in reilcc* f * *i Reduction by hydrogen (Kohlschuetter's tt'

ment

Generally

v nn

This process is of interest as giving an eh**rt Dissolve I gm of silver nitrate i i * free sol of water and precipitate with a slight t*?v Wash the precipitate of si]v^* caustic soda by repeated decantation with hot water, tii 200 c c of redistilled water, slm & suspend it k i t and then filter off any undissolved oxide

1-44 solution into a resistance glass flask 56 to 60 C in a water bath (or thermos! *iH of a current it bv hydrogen through pass Reduction is coi a tube of resistance glass 20 to 25 minutes.

SULPHIDE SOLSAs gold Other methods. most reducing agents, a methods of preparation of references to some of thesechlorideis

this chapter. Generally speaking, sols will result if gold chloride solutions containing about one part in 10,000 are treated with small quantities of the following reducing agents, in solutions containing from one part in 4,000 to one in 500 gallic acid,

This is an instance of a peptisation of a coarse precipitate 20 c c of Dissolve 0-5 gm of cadmium chloride water and precipitate with moderately concentrated ammonium sulphide. The precipitate should be a deep yellow and should settle rapidly if it does not , the ammonium sulphide solution requires diluting. Wash the precipitate by decantation with two or three lots of water, 50 c.c each, and suspend in 300 to 400 c.c of water. Pass a slow stream of hydrogen

Cadmium

Sulphide Sol

sol produced

by

sulphide through the mixture and shake occasionThe suspension first becomes milky, then ally. yellow and moderately clear, and after 20 to 25 minutes most of the precipitate will have been dis-

The sol may be filtered to remove any persed remains of precipitate, and boiled to drive off the excess of hydrogen sulphide, without congubtion occurnng. The filtered sol is a pale tjoldi 11 yd'ow

34

MISCELLANEOUS SOLSmarked greenishsol

in transmitted light, with cence in reflected light

opales-

Arsenic Sulphide Sol

ject of

This

has been the sub-

To prepare it, dissolve electrolyte coagulation. 2 gm. of arsenic tnoxide in one litre of watei keep After the latter boiling until solution is complete cooling the liquid pass a slow stream of hydrogen sulphide through it, with occasional stirring, until the,

many

classical investigations, especially

on

The colour does not deepen perceptibly pale orange colour in transmitted light, greenish-yellow opalescence in reflected light of 2 S can be removed by passing hydrogen the sol this must be done if the sol is to be

sol

is

with a Excess through used for

coagulation experiments

C.

MISCELLANEOUS PREPARATIONS.

Prussian Blue Sol. Dissolve 0-4 gm, of crystallized potassium ferrocyamde in 20 c c of water and Pour 0-4 gm of ferric chloride in 20 c c of water.the first solution into the second, slowly and without Allow the mixture to stand for a few stirring minutes and then pour it on a folded filter of hard The filtrate should be quite clear and run paper When filtration is complete, wash the fairly freely. precipitate with four successive lots of 25 c c of

water.

The precipitate is then dissolved in 300 c.c. of solution containing 16 gm of crystallized oxalic acid. The simplest way to do this is to pour the acid solution on the filter and allow it to percolate , the precipitate will be found to have been completely dissolved when the whole volume of acid has passed The solution of Prussian blue in through the filter oxalic acid is then dialysed in a parchment bag against repeated changes of distilled water, until the

FERRIC HYDROXIDE SOL.

last

35

batch of theis

latter gives

no perceptible oxalate

Owing to its deep colour and sensitiveness very suitable for cataphoresis and coagulation experiments For the latter purpose the sol can be diluted with an equal volume of water, as even in that dilution it is deeply coloured in a thickness of i cm The concentrated sol is quite stable, and there is, therefore, no reason for making a dilute sol directly, as this course entails waste of oxalicreaction

the sol

acid.

Heat 500 c c of water in beaker and, when it is boiling vigorously, add of a 30 per cent solution of feme chloride, The liquid turns a gradually and with stirring deep reddish-brown and remains perfectly clear The sol contains HC1, corresponding to 0-6 gm of FeCl a * & about 0*4 gm or approximately 22 milhmoles per litre. As this is a small fraction only of the HC1 concentration required for coagulation, the sol may be used for precipitation experiments without being dialysed, as well as for cataphoresis in the U-tube (no particles are visible with dark-ground illumination, so that the microscopic method is not Most of the HC1 can be removed by applicable). dialysis in the parchment bag, but only experience will tell how far dialysis may be continued without coagulation of the sol. Both the acid and the dialysed sol keep indefinitely Only the latter is suitable for experiments on the mutual coagulation of oppositely charged solsatall

SUSPENSIONS.Mastic Suspension This preparation is one of the classical subjects of investigation Dissolve o-i of powdered gum mastic in 10 c.c. of alcohol or acetone Pour the solution slowly into 500 c c. of Filter the water, stirring the latter vigorously

gm

suspension through a fairly close

filter

paper to

remove

coarser particlesis

almost opaque, with vivid pale preparation blue opalescence, in reflected light, and should be perfectly clear and a faint yellow in transmitted It shows an extremely bright blue Tyndall lightcone.

The

made with and tri-valent cations, suspension behaves somewhat differently from suspensoid sols (see chapter on Electrolyte " " Titration will be found somewhat Coagulation). on difficult, as there is no marked sudden change standing, however, the disperse phase separates very a as flocculent sedimentaclearly precipitate, though tion is naturally slow. In the U-tube a sharp boundary will be seen if observed in reflected light.HC1 and withas

Coagulation experiments should be

salts of urn-, bi-

the

using exactly the same The from other resins beginner will find dragon's blood convenient, as the colour is a vivid red Gamboge may be treated in the same way, A suspension which sjiows the Brownian movement, cataphoresis under the microscope, etc,, can also besimilar

suspension,

quantities,

may be made

38

SUSPENSIONS.

made by rubbing down

a stick of the gum with a few cubic centimeties of watci in a saucer (as is done with slicks of Chinese ink), diluting the resulting mixtuic with a large volume of water and filtering to

remove coaibcr

paiticles.

CHAPTER

V.

ORGANOSOLS.

THE most convenient method of directly preparing organosols of the noble metals is that of C Amberger, which wool-fat (lanolme) is used as protectiveagent

To prepare silver sol, dissolve 3-5 gm of silver nitrate in 5 c c of water and add this solution in very small quantities at a time to 15 gm of cold lanolme, incorporating it thoroughly with the latter by means of a pestle or a silver spatula The success of thesubsequent reduction depends on the completeness with which this is done. If any silver nitrate is left in the form of drops, the oxide and silver formed from them are of course not protected by the wool-fat and remain as a coarse insoluble residue when the an organic solvent. Then add latter is taken up in the same way a solution of i gm of sodium The mass turns first hydroxide in 5 c.c of water yellow and then brown, owing to the formation of On standing in the light the latter is silver oxide. the reduction is accelerated by reduced to silver gentle warming and by turning over the mixture from time to time, so as to expose the whole of it to the light. After about six hours reduction is generally complete, and the product is dissolved in 50 c c. of chloroform Fifty c.c. of petroleum ether and about 25 gm of fresh granulated calcium chloride are then added the latter to remove water, etc and the mixture allowed to stand for five to six hours. The solution, which should be a clear reddish-brown

40

ORGANOSOLS,

when diluted with about 20 volumes of solvent, is then poured off the solvent may be allowed to evaporate, leaving a mass of colloidal silver in woolThis disfat of the original salve -like consistency. solves easily in ether, petroleum ether, also in fattyoils

and

in paraffin.

Organosols of gold, platinum and metals of the platinum group may be prepared in similar fashion, for which the original papers should be consulted

EMULSOID SOLS AND GELS

ACID SOL AND GEL

is a solution of having a density 1-16, made by commercial water-glass syrup with The ratio of syrup to distilled water

starting material

water

sufficient for

The

by preparing a small lot, determining the density with a spindle be prepared in large quantities, may and should be kept in a bottle closed by a rubber stopper or a glass stopper well rubbed with vaseline To prepare a sol, dilute 30 c c of concentrated hydiochloric acid (1*2 sp gr.) with 100 c.c. of water, and pour 75 c c. of the sodium silicate solution into the dilute acid, The mixture is dialysed in a parchment bag against repeated changes or against the beginner will find the former running wateris

best ascertained

solution

coursefarit

moreis

satisfactory.

Experience will show

how

possible to push dialysis without the sol setting to gel prematurely in the dialyser. The sol should be perfectly clear and colourless It will keep for a length of time which can be ascertained only by experience as the removal of gel, formed accidentally, from flasks or bottles with narrow necks is inconvenient, sol under examination should be kept in wide-mouthed bottles or taper beakers. Setting is greatly accelerated by C0 2 carbonates,, ,

phosphates,

and

free

alkali.

The

effect

can be

demonstrated by bubbling

C0 2

gas through the sol

42

SILICIC

ACID SOL.

until the bluish tinge, which indicates the beginning or by adding small amounts of of gelation, appears dilute solutions of carbonate, phosphate or ammonia to the sol, gradually and with constant stirring, which is discontinued as soon as the sol appears bluish. If the solutions are too concentrated, or are added too rapidly, local coagulation and flocculation may occur instead of complete gelation.,

The concentrations given above

and

are fairly high

If will be found useful if a stifi gel is required the sol alone is wanted and requires keeping for some time, the same quantities of hydrochloric acid and of silicate solution should be used, but a larger volume of water. To determine the amount of SiO 2 in a given sol, evaporate 5 c.c. slowly in a weighed crucible to dryness and then ignite until the weight is constant In the later stages of drying gelation may occur, and the steam bubbles formed in the gel burst violently and may scatter some of the material, unless drying

proceeds very slowly The effect of lyotropic additions is the same as in the case of other emulsoid sols This can be shownqualitatively by placing 10 c c of freshly dialysed sol in each of three test tubes, keeping one as blank

and saturating the others respectively with sodium The sulphate and with ammonium thiocyanate sol containing Na 2 S0 4 will set before, and that conNH latter the blank the CNS after, taining 4 sample,

very generally does not set at all All vessels, measures, etc used for sodium silicate or silicic acid sol should be washed immediately and,

thoroughly.

GELATIN AND AGAR SOLS AND GELS.

" " Gelatin occurs in commeice as leaf gelatin in sheets about 9" to 10" long by 4" to 5" wide, showing

GELATIN

SOL.

43

the diamond-shaped marks of the wire netting on which the leaf has been dried as powder, and as foil without any of uniform thickness about 0-15 marks. The most suitable brands for practically all " Photothe work to be described are Coignet's " " " and First Quality," and Nelson's Crystal graphic " Leaf Since different brands differ appreciably in their physical constants and in their ash content, it is essential to start any given investigation with If an amply sufficient stock of the brand selected great constancy is aimed at, it is desirable to take leaves at random throughout a one-pound package, or to shear through the entire package and use the strips so obtained rather than the necessary number,

mm

of adjacent leaves

reproducibleat, sols

purposes, i e in all cases in which only and not quantitative results are aimed and gels containing a definite amount to a given volume of water, eg, 10 gm of gelatin to 100 c c of water, are quite suitable and are easier to prepare than sols containing a specified amount in a definite volume of sol The leaf is broken into pieces

For

many

" preferably not larger than f square, placed in a beaker, and the requisite amount of water poured on, care being taken that the whole of the leaf is covered , air bubbles should be removed by shaking or stirring. The gelatin is then allowed to swell, either to complete saturation, or for Bany arbitrarily fixed period, which, however, should not be less than two 01 three hours Complete swelling may take 24 hours or as gelatin imbibes something like ten even more times its weight of water, there will be no loose or unimbibed water, if the amount originally put on was less than ten times the weight of gelatin leaf.,

The thickened edges

of the leaf

take considerablyto soften

longer to swell than the rest,

and care should be

taken that the time allowed them completely

is sufficient

44

GELATIN SOLThe next operation

bear in mind that the 'properties of a gelatin gel or sol are not merely functions of the concentration and temperature, but depend on its whole vizthe solit

is the dispersing of the gelatin, which should be carried out on the water balh temperature of 35 to 45 C. is sufficient, but higher temperatures may be used to accelerate the process and for other reasons Thus, if the sol is to be filtered (see below), it will be advisable to heat up to 80 or 90, as otherwise the viscosity is high and the rate of filtration excessively low It is necessary to

previous history, , the period allowed for swelling, the temperature at which was formed and the length of time during which

was exposed

to

this

temperature

To eliminate

differences in the history, the practice is sometimes adopted of heating the sols for a definite time, say five minutes, to 100, at a definite rate to the

cooling

temperature at which the sol is to be used (e.g for viscosity measurements) and keeping the sol at the lower temperature, likewise for a definite time, before use. While this treatment goes a considerable " way towards obliterating the thermal history," it is yet safer to adopt a rigidly uniform procedure in,

gel cylinder is removed by dipping them into boiling water and allowing the gel to drop into an ample depth of cold water. Other shapes, e.g., prismatic ones, can be

closed at one end,

made by pouring the sol into suitable moulds thus can be made by using glass or metal tubes cylinders;

rently complete, as the modulus and the accidental birefringence do not attain their final values before that time. Bodies of can be gel of definite

filtration is ready for use when the gelatin is completely If the gel is wanted dispersed. especially lor the study of its elastic or optical properties, it must not be used for at least four hours aftei setting is appa-

and

any particular investigation. The sol in many cases does not require

shape

from which the

GELATIN SOLcast in

45

moulds made from heavy tin or lead foil, or wooden moulds lined with ordinary tin foil, which is rubbed with vaseline, any excess being removed byIn all cases the gel should wiping with cotton-wool be left in the mould for several hours after setting, as mentioned above In many cases, and always when a salt capable of forming a precipitate with calcium salts, chlorides, sulphites or sulphates has to be added, the sol will require filtering The most suitable paper is Chardin's, either the original brand or an imitation made in England It can be obtained in sheets or as folded niters, which, however, are too large for the small Folded filters should be batches usually required as the made, great care being taken with the point paper is rather thick it is not advisable to try to make more than twelve folds. A hot-water funnel is used those usually obtainable have the defect that the spout of the glass funnel is much too long, so that cooling and even setting may take place in the portion which passes through the stopper of the waterTo obviate this, a rather thin stopper, not jacket more than f ", should be used, and the spout of the cut off so as just to project through the funnel glass The temperature of the water bath should stopper. not be higher than is necessary to secure a reasonable this varies considerably with rate of filtration different brands of gelatin and, when solutes are of the latter. the nature with present, " " " hard and soft/' the Gelatins are classified as former type being desirable for most investigations " " The term hardness denotes a complex of qualities, " " " " among which are high melting and setting The melting temperature and high elastic modulus and setting points are, of course, not strictly defined, and can be determined and compared only by conventional methods. An apparatus suitable for this purpose is illustrated in Fig. 7. A test tube A is; ,,

46

GELATIN SOLthe centre of a suspended 300 or 400 c c beaker B, which serves as a water bath, by

means of the guide C, through A which it must slide freely tube f" diameter X 6" long is suitable it is weighted with,

FIG

gm of mercury, It essential that the tube is should be perpendicular when if the rim it is resting on C is not sufficiently regular to ensure this, a square collar, say of rubber, should be used and permanently attached to A glass rod D, the tube about f" diameter for a f" is tube, suspended exactly in the axis of the test tube (If the apparatus is to be used it advisable is to frequently, mount it permanently to ensure correct alignment.) To determine the melting point the test tube is filled with a definite quantity of the gelatin sol under examination, the beaker filled with water at a definite temperature, say 15 C., and the sol allowed to set for a definite The rod, with the test time tube hanging to it, is now raised a definite height (which the illusstage is shown tration), and the temperature of the bath slowly raised, with constant stirring, until15 to 20,

MELTING AND SETTING POINT

47

the test tube slides off the gel cylinder surroundThe temperaing the rod and comes to rest on C " ture at this moment is noted as the melting " " " If the is also to be deterpoint setting point mined, the rod is lowered to its original position, the flame extinguished, and the bath allowed to cool The rod is raised very slightly from time to time, until it just lifts the test tube with it, the tempera" " ture at this point being noted as the setting point It must be remembered that there is considerable hysteresis and that the setting point of harder brands may be as much as 7 or 8 C lower than the melting point of about 10 per cent gels

A more delicate method of determining, with very simple means, the setting point is based on the wellknown fact that the exposed surface of a gelatin gel which has been allowed to set quietly is not smooth like that of a hquid, but shows a network of wnnkles The formation of these wrinkles is not due to drying, but occurs actually during the last stage of setting The alteration the appearance of the suiface is very striking if it is observed under an acute angle in

reflected light, and it may be used for determining the setting point in the following manner small porcelain crucible is filled with about 10 c c of sol and the bulb of the thermometer completely immersed The reflection of the window in the in the latter

surface is then observed, attention being fixed on some dark object in the light field, such as the windowThe reflection of such an object frame or the like is, of course, distorted by the menisci formed by the at of the the wall crucible and the stem of the sol thermometer, but is a smooth and unbroken curve. As soon as wrinkling commences, the image is broken up into fringes (see Fig 8, a and 6) the fall of temperature between the time when this alteration in appearance becomes barely perceptible and when it is quite unmistakable rarely amounts to,

48

GELATIN GELis

more than 0-1, which

a more than

sufficient

accuracy As regards other physical properties of the sol, the one most likely to require investigation is its viscosity at different concentrations and temperatures

The methods

to be employed are described fully in the chapter dealing with viscosity measurements. Since viscosity is particularly sensitive to variations

AFIG, 8

" in the thermal history," -uniformity of procedure in the preparation of sols for this purpose must once more be insisted on as being of fundamental importance. As regards the gels, reactions in gels are treated in a separate chapter. The quantitative study of the modulus of elasticity or the accidental birefringence produced by strain is beyond the limits of this bookIt is, however, easy to demonstrate the latter by very simple apparatus, if a Nicoll and selenite plate are

GELATIN GEL.available,

49of the strains

and the study, particularly

set up by drying, is instructive The apparatus is simply an open box (Fig 9) about 16" high X 8" wide Two glass plates photographic plates from which the film has been removed are suitable rest on the bottom, inclined under an angle of about 53 with thelatter.

strip

of

either

ground-glass or glass coated on the lower side with the " " matt varnish used

photography rests on two ledges, about g" or 10" from

An top of the box. opening in the centre of the the takes mount of the top Nicoll prism, which can be If light from a lamp rotatedthe

placed as shown is reflected from the double glass plate, a sufficient fraction of it is polarized to show very slight strains in gelatin gels containing

10

per

cent,

and

more The

strains set up during drying and their progressive changes can easily be traced

^^' ""-

and are instructive

body

of gelatin gel, unless it is a simple surface of revolution

FIG 9 approximating fairly closely to a sphere, does not remain similar to itself during drying, and if the surfaces meet in edges very considerable distortion occurs Thus, a right cylinder with flat ends has two circular edges, and drying is atfirst

much more

rapid along these than

it is

on the

50

AGARflat surfaces.

SOL.edges, therefore, con-

curved ortract ends.

The

and the cylinder becomes a barrel with convex The edges have now become so dry and rigid

that very little further drying takes place in them, while the rest of the surface is rapidly shrinking, and the final shape is a single-shell hyperboloid with concave ends Similarly, when a cube is allowed to dry, the edges contract first and the faces become while the final surface has concave faces with, convex, of course, concave edges The distribution of strain, and the change from compression to tension, caneasily

be observed and analyzed.

Gelatin being highly liable to putrefactive changes, neither sols nor gels can be kept for long without sterile precautions, which are beyond the scope ofthis work.

Hardening agents like formaldehyde alter the physical properties of gelatin so much that they are suitable only for preserving finished specimens Directions will be lound in the chapter on theLiesegang phenomenon A gar occurs in commerce as strips having a fibrous texture, as a fine powder, and as bars of square crosssection. The first-named is the cheapest form powdered agar has the advantage that the time If necessary for swelling is considerably reduced. strip is used it is torn into small pieces, which are allowed to swell the requisite volume of water for A small addition one part in about 24 hours of is usual and promotes imbibition, acetic acid 500 but is not essential. The mixture is then boiled slowly, until the shreds have entirely disappeared. The sols are always turbid and show even macroscopic fibres and fragments, so that they must at least be strained through fine muslin or through If clearer sols and gels glass or cotton-wool plugs. are required they must be filtered through Chardin in the manner for gelatin the water described paper in the jacket must be boiling With sols containing;

AGARi per cent,

SOL.

51

filtration is tediously slow, and the filtrate sets long before the filtration of even small batches is complete. If a large drying oven or

and over

kept at 100 C. is available, the most convenient course is to place the whole apparatus, ie., filter funnel and beaker or flask for the filtrate, into it, when the whole can be left to itself without further The setting temperature of agar sols is attention between 35 and 40, while the melting point of gels lies between 90 and 100, so that gels have to be heated on a water bath at boiling point to obtain a are not liable to putrefaction and are, sol Agar gelssterilizer

therefore, preferable to gelatin for long-continued which the specific experiments e g on diffusion They properties of the gel are of no consequence are, however, a good medium for the growth of various moulds and occasionally of Bacillus prodigwsus, Both occur chiefly which latter forms red patches on the surface, and the rest of the gel may generally be used after the affected patches have been cut,

away.

Agar gel, unlike gelatin, does not adhere to glass, and specimens may be removed from moulds without the heating necessary

the case of the

latter.

Thus cylinders may be cast in tubes stoppered at the bottom, and will drop out when the stopper is removed. A certain amount of liquid, which also contains agar, exudes from agar gels on standing, partly on the surface and partly between the gel and This is a normal phenomenon the containing vessel and does not indicate faulty procedure in the preparation. It is desirable to demonThe Lyotrofiic Series strate the general nature of the series by showing its effect on two sols as chemically different as gelatin and agar. Sulphates, chlorides and thiocyanates may be chosen as representative specimens, sufficient of each being placed into a loo-c.c beaker to

52

THE LYOTROPIC

SERIES.

for

produce a concentration of N/2 in 50 c c. of sol (allow water of crystallization '). Each of the beakers so piepared now receives 50 c c. of sol, 10 per cent, gelatin sol and I per cent, agar sol being suitable, and the same quantity of pure sol is placed into a All four beakers are fourth beaker for comparison placed in the water bath until they have attained the same temperature and are then taken out and The water bath should be at 35 allowed to cool to 40 C. for the gelatin sol and at boiling point for the The order in which the gels set will be the agar sol same for gelatin and agar, and the intervals between the four specimens will be considerable the sol containing thiocyanate remains liquid at room tem;

perature

The

effect of

the lyotropicalkali,

dilute acid may also be

and

demonstrated very simply m the followSquares having a side of, say, 15 ing manner are cut from the gelatin foil mentioned above if this is not obtainable leaf may be used, but the diamond markings, which are highly strained, should be avoided The squares are placed in watch-glasses or Petri dishes containing a few cubic centimetres of the following solutions N/5o HC1, N/SO NaOH, 4 CNS and water N/i Na 2S0 4 N/2 NaCI, N/2 The squares should be held in a small forceps and immersed quickly and completely, without allowing The difference in air bubbles to adhere to them swelling will be quite noticeable after one hour (when the gelatin placed in the thiocyanate solution is probably completely dispersed), although complete It must equilibnum is not attained for many hours be remembered that the foil swells in all directions

series, and also that of on the swelling of gelatin

mm,

NH

and that the increase

in volume is, therefore, proThe squares are portional to the cube of the side. best examined by holding the watch-glass 2" or 3"

above a

"black

background,

when they appear

PURIFIED GELATINturbid, or of courseliquid

53

by

carefully pouring off the

The work described so far can Punfied Gelatin be carried out with the raw material obtainable comThis always contains electrolytes, which mercially it may be necessary to remove as far as possible, although it should be noted that the physical properties of the gelatin are sensibly affected by the

A known prolonged washing which is required quantity of the leaf is placed in a weighed tall beaker, capable of holding a volume of water equal to at least 15 times the weight of gelatin Running water is then passed in near the bottom of the vessel and allowed to overflow for 48 hours If an adequate supply of distilled water is available it may be usedfailing this,

which tap-water may be employed, must be completed with seveial changes of distilled water. To prevent the formation of mould a few fragments of camphor or thymol, wrapped in muslin, are added between the leaves, s"o as not to escape with the water Since gelatin imbibes something like 10 times its weight of water, sols of greater concentration cannot be made directly from the washed gelatin by warming To obtain them it is necessary either to dispersion or to to dry the wet mass over 2 S0 4 or CaCl 2 evaporate the dilute sol to the required concentration at fairly low temperature, the concentration in either case being determined by weighing The second procedure affects the sol perceptibly, especiallycase washing

if

Amount per Volume. only with sols and gels containing a definite peicentage of gelatin to a given amount of water; in other words, with sols containing a definite amount of gelatin in a given weight of sol. The preparation of sols containing a definite amount of substance in a given volume is complicated chieflySols-i

prolonged '

*"'

We ha\

Definite

54

GELATIN CONCENTRATION

fact that sols are not liquid at the tempera tuxes for which the usual measuring vessels an graduated, and the first point to decide is whethe the sol is required to have a definite concentratioi

by the

some particular temperatuie, say 35 for vis cosity measurements, or at some arbitrarily chosei lower temperature, at which it may be transformec In the former case the flask to be usec into gel should be filled with water at the standard tempera ture and then placed in a water bath and wanned tc a fresh mark should be the temperatuie selected placed at the level reached by the water. The volume to this mark is calculated from the ratio of the specific volumes of water at the two temperatures selected for 15 and 35 C respectively these arc, foi instance, 1-00085 and 1-00586, so that the volume of, say, 500 c c measured at 15 will be 500 x 1-00586 == 502-5 c c. at 35,at, ,

1-00085

noted and the gelatin content calSince it is inconvenient to soak and the leaf in a long-necked flask, this should disperse be done in a beaker with about 75 or 80 per cent, oi the total volume of water required, the more concentrated sol thus obtained poured into the flask, which is placed in a thermostat at the required temperature, and the beaker washed out with successive small portions of warm water, which are transferred to the flask until the mark is reached. The contents of the flask, of course, require thorough mixing before use Commercial leaf contains a considerable amount of which figure culated on itis

moistme, rarely less than 10 per ceni., which must be taken into account. It can be removed almost entirely by drying at 100 to constant weight (note that gelatin takes up moisture from the air even during the time required loi weighing), which treatment, howevei, affects the properties of the material

IN

^A

GIVEN VOLUME

55

It should, therefore, be applied very considerably only to a small sample, the rest of the material being kept in an airtight receptacle from the time at which the sample has been taken, so that its moisture content remains constant. When making up sols to a given concentration per volume, this should, of course, be calculated on the weight of dry gelatin It should be remembered that sols made by diluting a more concentrated sol differ slightly in

their physical pioperties from sols produced by dispersing the gelatin at once in the total water required To obtain comparable results it is again necessary to observe the rigid uniformity of procedure which has already been insisted on, i & to use the same percentage for diluting, to have the added water at the same temperature as the sol, etc If powdered gelatin (the usual brands of which are,,

hard than the best however, markedly less brands of leaf) can be used, the procedure is simpler, since there is no difficulty carrying out soaking and dispei sion in the measuring flask itself The latter should be about half filled with water by means of a long (thistle) funnel, so that the neck remains quite dry. The powdered gelatin is then poured in, in small portions and in a thin stream, through a widenecked funnel, and the flask shaken frequently to cause the powder to sink without the formation of lumps. On no account must the powder be placed in the dry flask first. When the whole of the powder is submerged further water is added to within about p 5 c.c. of the mark givmg the volume at 15 C. (this for other sizes the margin refers to a 500-c,c "flask should be in proportion), and the necessary time allowed for swelling. The flask is then placed in the water bath, and the volume is made up to the mark giving the volume at the working temperature which Careful mixing before use is has been fixed upon also required in this case, since the gelatin does not

"

"

56

GELATIN

SOL.

diffuse perceptibly during dispersion, so that a concentrated layer of it rests on the bottom.

EGG ALBUMIN SOL

THE only commercial raw material is dried egg albumin, and the beginner should carry out the experiments described below with a sol made from it although its use is open to objections, the results obtainable correspond sufficiently closely to those " " albumin natural recorded in the literature for Crush 15 gm of dried egg albumin coarsely and introduce in small portions, with stirring, into 100 c c of water. The albumin at first adheres to the walls of the vessel with great tenacity, but is The sol easily detached as imbibition proceeds should be stirred from time to time and lumps,

broken up until dispersion is complete. The sol is turbid, with a varying small fraction of insoluble matter, which does not settle even on prolonged It must be filtered, preferably overnight, standing through asbestos in the manner described on p 12 filtration through paper is extremely tedious and,

involves considerable loss. The filtrate from asbestos is a yellowish liquid, opalescent, but quite sufficiently clear in moderate thickness, say in test tubes 15 or 18 mm. diameter, to allow even incipient preAlbumin sols are cipitation to be noticed easily very liable to undergo decomposition and should be in warm weather a trace of thymol used quite fresh may be added to the mixture before filtration without,

affecting the properties of the sol Place a test tube containing Heat Coagulation about 10 c c. of sol in a small water bath and heat

slowly,

stirring

constantlyat

with

Note the temperature

which the

sol begins to

a thermometer. turn

58

EGG ALBUMIN SOL

Removethe test tube from the

white and opaque

bath, add water and break up the coagulum, to show that it does not disperse again, i e , that the heat coagulation is irreversible. Irreversible Change by Adsorption Albumin is readily adsorbed at the interface between sol and another liquid, and becomes insoluble in the process This is easily demonstrated by placing in a test tube 10 c c of sol, adding i c c of some organic liquid heavier than the sol, e g., chloroform or carbon tetrachlonde, and shaking vigorously, so that added These sink to liquid is broken up into small drops, the bottom and remain perfectly separate, which shows the formation of a film preventing their the sol, as no coalescence and evidently insoluble

change takes place even on standing for some time To demonstrate that the adsorbed film is also insoluble in water, pour off the sol and replace by no coalescence occurs even then water Salting-out and the Hofmeister Series of Anions It will be sufficient to try a few of the more characteristic salts of the seiies by adding the dry salts to the same volume of sol, so as to keep the albumin The molar concentration approximately constant concentrations necessary to produce immediate tur" " sols albumin sols natural in containe,, (i bidity ing, like ours, the other constituents of egg-white) are given below, as well as the amounts of the most readily obtainable salts required to produce these concentrations in 10 c c. volume;

MNacitrate.

per

liti

Gmcitrate.

in 10 cc of solution

0-56

1-647 crystallized neutral sodium

Place the coarsely powdered salts in flasks with a mark at 10 c c or, if these are not available, in test tubes 15 or 18 ram diameter which have been proPlace the flasks vided with a mark at that volume or test tubes in a water bath kept at about 35 and dissolve the salts gradually by slowly reversing the tubes at intervals they must not be shaken, as the formation of froth is to be avoided Watch the appearance as solution proceeds and note that a marked turbidity appears only when the whole of the salt has gone into solution, except with thiocyanate, which does not salt out even in saturated,

solution

above, and in another 3 gm of ammonium thiocyanate, fill to the 10 c c mark with the acid sol, and dissolve the salts gradually No precipitate is formed in the sol containing the sulphate, while the sol conbecomes turbid and eventually taining thiocyanateclots

Dilute the turbid sols with an equal volume of and note that they become clear, i e the salting out is reversible Reversal of the Hofmeister Series in Acid Sols Acidify 20 c c of sol by adding i c c of normal Place in one test tube the same hydrochloric acid as use<i ) quantity of sodium sulphate (2-577water,,

completely

PrePrecipitation by Salts of the Heavy Metals, pare some 2N solution of copper sulphate (say c of the salt in c. of 50 12-480 gm crystallized Add from a burette a few drops of this solution) solution to 10 c c, of sol a heavy greenish coagulum forms immediately Continue to add copper suloccasional with solution, phate stirring ; the precipitate le-dissolves and has disappeared when 10 c,c> of solution has been added, i e., when the mixture is;

normal20 c c

sufficient

Now add respect of copper sulphate, powdered coppei sulphate to saturate the a second (about 5-6 gm.) and dissolve;

60

PURIFICATION OFwhenthe solution has become

precipitation begins

saturatedis trreversible,

To show that coagulation by salts of heavy metals repeat the experiment as far as adding a few drops of copper sulphate solution to 10 c c of sol, then dilute with water, and note that the coagu-

lum does not dissolve. The student desirous of working with pure albumindo well to practise the usual methods of purificaThe the first instance with dried albumin methods are based on the fact that the constituents viz of white of egg other than albumin, globulin, ovomucoid, etc are salted out by lowci concentrations of ammonium sulphate than is albumin Disperse 15 gm of dried egg albumin in 100 c c of Add water, as described, but do not filter the sol small portions sufficient finely powdered pure ammonium sulphate to produce a half-saturatedwill

tion

solutionsolved.

38 to 39

gm

is

required

Each addition

should be

forms and is removed by The filtrate filtration through a folded paper filter contains the albumin dispersed in half-saturated can The salt solution now be ammonium sulphate removed by dialysis in parchment, or, better, collodion against running water, and a moderately pure The usual method is, howalbumin sol obtained ever, to precipitate the albumin by saturating the solution with ammonium sulphate, a further 38 to 39 gm being required for every 100 c c of filtrate. A thick coagulum of albumin forms, which is filtered overnight and allowed to dram as far as possible. The residue, which always contains a considerable amount of mother liquor, is then dissolved in the smallest volume of water which will give a clear sol,

A white coaguluni

made only

after the previous one has dis-

and dialysed as explained above to remove ammonium,

sulphate

The same method

is

applied to fresh white of eggs.

EGG ALBUMIN SOL

About 28;

61

to 30 c.c of white can be obtained from average fowls' eggs this contains about 10 per cent of albumin The total protein content is about 12*2 per cent the difference being accounted for by The egg-white is beaten up globulin and mucoid with an equal volume of saturated ammonium sulphate solution, which produces half-saturation in the mixture and precipitates the latter constituents The coagulum is filtered off and the filtrate saturated with ammonium sulphate to precipitate the albumin, which is filtered off and dissolved in a small volume of water. This sol is again precipitated by saturation with ammonium sulphate, and the previous,

several re-precipitations are operations repeated The last coagulum required to obtain pure albumin is dissolved in a small volume of water and dialysed The losses are fairly conto remove the sulphate siderable, and the albumin content of the sol finally obtained after dialysis will be between 2 and 2-5 gm. of albumin for every 30 c c of white used originally. Although the general student will hardly have occasion to use it, the classical method of making " " It may be is here given. crystallized albumin tried with the white of two or three eggs, which should be perfectly fresh. The volume of egg-white is measured and an exactly equal volume of saturated ammonium sulphate solution added to it in small portions at a time, the mixture being vigorously beaten with an egg-beater after each addition until This is the whole has been reduced to a stiff froth allowed to stand overnight, and is then filtered to remove the coagulum of globulin, etc Ten per cent diluted to 10 acetic acid, i e glacial acetic acid times its volume, is then added to the filtrate from a burette, a single drop at a time, with gentle stirring to re-dissolve the precipitate formed locally before a further drop is added. This is continued until the solution becomes permanently turbid the exact;,

62

CRYSTALLIZED EGG ALBUMIN

degree of turbidity can only be found by practic but must amount to something more than me;

When this point has been reache* opalescence i c.c of acid for every 100 c c of solution is added. copious precipitate forms, which, on standing an occasional gentle shaking, becomes (micro-) crysta to obtain the full yiel line after five or six hours it should, however, be allowed to stand for 24 hour! The precipitate is filtered oil and dissolved in a sma volume of water the solution is then dialysed, 01 if further purification is desiicd, it is again preci This is done by dissolving the coaguluu pitated from the filter in the smallest possible volume o water, acidifying with a few drops of 10 per cent acetic acid, and then adding concentrated ammomun sulphate until a slight permanent turbidity lesults After 24 hours' standing the bulk of albumin has beei re -precipitated. The beginner will find the exact degree of turbiditj,

required somewhat difficult to judge and must be In dialysing albumn prepared for disappointment sols remember what has been said on page 27 regardsols which exert an ing appreciable osmotic pressure and use suitable anangements.

LITERATURE,This is too voluminous to allow of being summarised. Students must consult the text-books on Proteins or those of Biochemistry.

CHAPTER VIII

EMULSIONS.as icgards methods of preparation and properties these fall into two classes, which are bevt the pure oil-water emulsions, in studied separately which no solute is present in the water, and the con-

BOTH

centrated emulsions, which can be produced only by adding to the water phase certain substances which greatly lower its surface tension and occasionally possess other properties as well. Pure Oil-Water Emulsions These are most conveniently prepared by the following method o-i c.c. of the oil (which may be a paraffin oil of low viscosity, oleic acid, or generally any other liquid immiscible with water, but soluble in alcohol) is This dissolved in 10 c c of alcohol or acetone solution is blown from a pipette into one litre of the water is well agitated before immersing water the pipette, the point of which should be 10 to 15 cm. below the surface. The resulting emulsion should:

show a bluish tinge in reflected lig^t well marked with oleic acid), and be-

y~~<-<;.,

Yrly*

il\

ji

in transmitted light.

The emulsion should be examined with a darkground condenser and the signof the charge deter-

mined in the cataphoresis apparatus. The coagulation by HC1 should be watched under the microscope as the phenomenon, viz., coalescence of dischargedparticles to bigger ones, with decreasing amplitude of Browman movement, is slow and more easily

followed than with suspensoids, with the behaviour of which it otherwise agrees

64

EMULSIONS.HC1

Electrolyte coagulation should be tried with

,

and with, say, CaCl a and A1 2 (S0 4 ) 3 salts of umvalent cations act only in very great concentrations. The effect of the coagulant shows itself macroscopically by the disappearance of the bluish opalescence, the emulsion becoming whitish and turbid instead Samples should be taken at intervals and

microscopically (ordinary illumination, sub-stage condenser and a fairly small diaphragm, magnification about 600 diameters), when it will be found that globules about 3^ diameter gradually take the place of smaller ones, this being the size at which Browman movement becomes so sluggish that further collisions between globules, and therefore formation of larger ones, practically cease to occur The emulsions to which sufficient coagulant has been added gradually clear from the bottom upwards, " " provided the oil has a density lower than that of water. The rate of clearing should be measured at convenient intervals, 24 or 48 hours, according to the difference in density, and the size of the globules calculated from Stokes 's formula (determine density

examined

using

of oil to three decimals) Concentrated Emulsions- To prepare these it is necessary to lower the surface tension of the aqueous phase, the most convenient agent for the purpose being a soap. Either a soap solution may be used, the preparation of which will be described below, or the soap may be actually produced in the process of This method, which of course is emulsriication applicable only to oils which are glycendes, consists in shaking up the oil with a dilute solution of caustic soda, N/SO to N/ioo being suitable concentrations. Small quantities may be prepared in test tubes , solution into a test tube of pour 10 c c of the 25 to 30 c.c. capacity, then add ordinary olive or cotton-seed oil in lots of i c.c., close the test tube,

NaOH

R'VTIO

OF PHASES

(^

with ihe thumb and shake vigorously aftei each addition. The emulsion becomes a pine white (whv /*}, and aftei (he addition of about TO c c of oil the viscosity inn eases so much that the dispeision of Hither oil becomes difficult Laiger quantities may be pi epaied in the same way in any shaking apparatus which may ho available in this case, too, the oil should be giadually added in small portions,i,

The

oil

a sluip boundni y forming between the concentrated emulsion at the top and the dispeision medium, which is tm bid owing (o the piesence of soap and very line pail ides. This lise continues until the oil globules ate in closest packing, as they aie not of unifoim sixe, no exact figure can bo given for the percentage of disperse phase, but it will be found to be 70 per cent 01 over. The volume ratio can be determined, with small crrois due to contraction, etc., in the following way a burette is filled to the lowest mark (i e the one bearing the highest numbei) with dilute hydrochloric acicl The emulsion IB then poured into the burette, the volume noted, the burette closed with the thumb, and emul,

in emulsions thus pieparecl gradually rises,

ralttd

sion and acid thoioughly mixed. The oil separates and uses any small globules which may remain sepaItom the main bulk must be made to unite;

with

it

by tapping andofoil is

inclining the burette,

The

then read off and the volume of continuous phase obtained by difference. When mineral oils are to be emulsified the procedure described ib not applicable, but soap solution

volume

must be employed, Ammonium oleate is extremely HluMeious, but is not obtainable eonmiei dally, andsodiumolenle(oliveoil

or

Marseilles

soap)

will

li is cut into fine genes ally have to bo used, shavings, which aie allowed to dry in all for three or four days, and 10 gm of the air-dried material dissolved in OIKS lit us of distilled water, at 30 to

66I

SIMPLE APPARATUS FOR

The solution is allowed to 40 C stand in the cold for 24 hours and then filtered twice through the samefilter of fairly

'

Emulsificationplished

open paper. may be accomas

byII

shaking,

describedis

above

no shaking device

available, fair quantities prepared in the appaiatus

may beshown

Fig

10,

which

may

be made up

from

vessels

to be found in the

A tall cylinder is laboratory closed by a rubbei stoppei with

twopei lorations

A thistle funnel,

diameter, of reaching to within 3 or 4 the bottom of the cylinder, passes through one of the perforations a

having a tube 3 to 4

mm

mm

ball tube, with

two

or three balls,

A large pipette through the other with its lower 50 to 100 c c end drawn to a capillary point, is suspended above the thistle funnel so that the point touches the wall of the funnel. The point is made so fine that 50 c c of oil of low viscosity, like the paraffins used in lamps, takes 25 to 30 minutes to empty If the flow is found too rapid it may be reduced by fitting a short length of rubber tubing, provided with a screw clip, to the upper end of the pipette. The apparatus is used as follows The stopper is removed and aknown volume^

of

poured into the cylinder

soap solution it should

;

FIG 10

not exceed one- third of the total

PREPARING EMULSIONS

67

volume. A few drops of the oil to be emulsified are then poured down the funnel and the latter rotated slowly, so that the whole of the tube is wetted by oil The stopper is then replaced, the pipette filled with oil suspended as explained above, and the ball tube connected to the filter pump. The latter should be so adjusted that the air issues in a uniform string of separate bubbles at the bottom of the funnel tube When the pump is working properly, the clip at the top of the pipette is opened to the required extent, and the apparatus The oil, which then requires no further attention runs down the tube in a very thin film, is broken up out at the lower air bubbles when the passing by edge of the tube, and thorough emulsification takes Frothing is rather marked at the beginning, place but subsides after a little oil has been emulsified, and the ball tube prevents froth from being drawn into the suction tube to any extent The emulsions made with soap solution separate a " cream," like those prepared with caustic soda, and the same methods may be used for determining the

volume ratio Emulsions which do not separate, whatever the volume ratio, can be made from oils which have the same density as the dispersion medium Thesimplest way is to prepare suitable mixtures of either olive oil, cotton-seed oil, or lamp paraffin with The carbon tetrachlon.de (density at o 1-632) ratio may be appioximately calculated from this and the density of the oil as, however, the co-efficients of expansion of the aqueous dispei sion medium and the mixture of oil and CC1 4 differ widely, exact equality can only be secured by experiment at a For this purpose the approxidefinite temperature mate mixture and a small beaker filled with the,

soap solution are placed in the thermostat, a i c c pipette filled with the former, and slowly blown out

52

68

EMULSIONS

under the surface of the soap solution, so that a single chop is formed; which can easily be detached from the pipette According as this diop uses or sinks, more CC1 4 or more oil is added to the mixture, until the drop remains practically stationary for a few minutes Emulsions of this kind are especially suitable for viscosity measurements (which see)LITERATUREFor recent papers, see article Emulsions in Second Report of British Association Committee on ColloidChemistry, 1918,p.

"

"

20

CHAPTER IX.

ULTRA-FILTRATION.given by H, Bechhold to a method separating the disperse phase of sols fiom the medium by means of filtration under dispersion pressure through porous membranes impregnated with gels, the permeability of which may be varied As considerable pressures may within wide limits have to be used metal apparatus is essential, which limits the applicability of the method to some An apparatus suitable for sols containing extent a very small amount of solid only (as is the case withof

THE name was

most suspensoidfiltering

sols) is illustrated in

Fig n.

The

perfoiated metal disc a, which is clamped between the body b of the filter and the slightly conical bottom c and rests on six The branch d, closed by a radial nbs in the latter screw cap, serves for filling the filter, and the necessary pressure is generated by a bicycle tyre pump connected to the valve e. The joint between the vessel and the cover is made tight by a rubber i ing cemented to the spigot on the foirner with marine To prepare the glue or Chatterton's compound filter for use, the cover with the perforated metal is removed, the membrane placed carefully on plate the cover is then replaced and the two the latter The filter is then charged through nuts tightened. the large inlet, the cap replaced and tightened, and pressure generated by means of the bicycle pump The filter should not be more than about half full, so as to leave a sufficient air space, as otheiwisc the;

membrane rests on a

7ofalls

ULTRA-FILTER.

too rapidly and the apparatus requires pressure The pressure continuous attention and pumping to be used depends on the denseness of the membrane to two or three atmospheres (30 45 Ibs per square inch) will generally be sufficient, but the;

w;^^\jA\\yIBt*

FIG ii

apparatus should be strong enough lor a maximum working pressure of five atmospheres, i.e it should be tested with eight atmospheres, as this adds very little to the cost The apparatus is supported on a tripod stand, and the beaker or flask for the filtrate placed below it.,

The membranes are made from circles of filter paper impregnated with either acetic acid collodion

COLLODION MEMBRANESor gelatin

71

hard filter paper, such as used for on Buchner funnels, must be brands employed equivalent to Schleicher and Schuell's Nos 575 or 602 are suitable. Collodion membranes are more convenient than the average their permeability, i e gelatin ones size of the particles which are ]ust retained, varies As it is with the concentration of the collodion difficult, without experience, to foretell what concentration will answer in any given case, a range of filters should be prepared impregnated with, say, sols of i. 2 3. 5 7 an d I0 P er cen t collodion, i e

vacuum

filtration,

collodion cotton containing i, 2, etc gm of cotton in 100 c.c. of sol, the solvent being glacial acetic acid The preparation of these sols has been fully described,

under "Dialysis," p 24If a number of filters are prepared with sols of various concentrations, the latter should be marked

The sol is in pencil on ike disc before impregnation poured into a small dish a porcelain developing a filter paper seized in a forceps dish does very well, near the edge, within the width which will eventuallybe covered by the rubber joint ring, and slowly immersed in the sol under a very acute angle with Care must be taken not to trap any air its surface. bubbles underneath the paper and to have it uniformly penetrated by the sol this is readily seen by the paper becoming translucent, like oiled paper, while spots not penetrated by collodion remain opaque and white The thoroughly impregnated disc is then slowly withdrawn from the liquid and held vertically above it to allow the excess to drain it should be turned to and fro in its own plane, off as otherwise a thick ridge is f earned at the bottom, which may prevent a good airtight joint being made when the disc is clamped in the apparatus. The discs are then submerged in water, which is constantly changed until all acetic acid has been washed;;

72out,

WO

OSTWALD'Sindefinitely in

and may then be kept

water

saturated with chloroform or camphor, to prevent the formation of mould. Gelatin sols containing from 2 to 10 per cent may be used instead of collodion, the sols being prepared in the usual way The vessel containing the sol during impregnation must be placed in a water bath, and the temperature chosen should be maintained constant and adhered to throughout, as otherwise filters made even with the same concentration will

vary considerably.to gel;

The

discs are impregnated

and

drained as described, during which time the sol sets they are then hardened in a cold solution of formaldehyde, 2 to 4 per cent., which is placed in a The discs are then rinsed refrigerator for 24 hours. water and can be kept under water saturated with or gelatin ultra-filters chloroform collodion either // are allowed to dry, even partially, they become useless. Two per cent collodion filters should give a colourless filtrate with Prussian blue sol (prepared as described on p while 3 to 4 per cent filters 34), should retain practically the whole of the disperse

phase in ordinary gold

or tannin).

sols (reduced

by formaldehyde

and the consequent contact with metal can be avoided by adopting either of the following methods of making ultraThe first one filters, which are due to Wo. Ostwald. furnishes membranes suitable for use with the filter " " pump, while the second one produces spontaneous ultra-niters, i e., membranes of sufficient permeabilityof special apparatus

The use

to allow liquid to pass simply by hydrostatic pressure. i Filters for Use with Vacuum The collodion used has the following composition collodion 2 or cotton, ether, 84 c c gm. alcohol, 14 c c. " " " the commercial Collodion P.B or Collodion, " methylated may be used. For preparing conical funnels, a circle of ordinary, , ;

ULTRA-FILTERSroughfilter

73

and placed

To ensure perfect to make one fold

paper is folded twice in the usual way a well-fitting, smooth glass funnel. fit it may sometimes be advisable

only at first, to place the paper the funnel, and to make the second fold when good contact all round has been secured by careful smoothing. The filter thus prepared is filled with collodion up to the edge when this has penetrated the paper over the entire surface, the excess is the filter emptied, being slowly turned while this is being done Turning is continued while the collodion dries superficially (the time required depends on the room temperature and the desired permeability, and may be from three to six minutes) when it no longer sticks to the finger on being touched hghfly, the funnel is placed in distilled water The filter is ready for use after about 15 minutes' immersion, but be under water may kept indefinitely provided chloroform or camphor is added,

The same method may be adopted for preparing Buchner funnels, but in this case a joint must be made between the edge of the paper and the perforated plate before impregnation, as otheiwise the collodion gets under the paper. A solution of 2 gm. of white crepe rubber in 100 c c. of petroleum ether is used for the purpose The funnel is inclined under an angle of 45 and a few cubic centimetres of the sol rubber poured down the side, care being taken that it does not reacjj the perforations. By turning the funnel round the sol is distributed to form a band

round the perforated " "

tackytion

area,filter

and whilepaperis

this is

still

the circle of

and squeezed down

all

round

placed in posiAfter a few

minutes' drying a second rubber band is produced in exactly the same way, and when this has dried the paper is impregnated with collodion, as described above When the collodion has dried to nearly the required extent a third rubber joint is made,

74

WO. OSTWALD'S

and the funnel placed

in distilled water, as before described The collodion for these 2 Spontaneous Filters contains 4 gm of collodion cotton to 12 c.c. of A circle of ordinary alcohol and 84 c c of ether. rough filter paper is folded twice in the usual way,

placed in a well-fitting smooth funnel, and thoroughly wetted with distilled water Any excess is poured off if a little remains in the or allowed to dram through filter it must removed with a twist ed the be of point The funnel is then partly filled spill of filter paper,

with collodion, which is spread uniformly by inclining and turning, the excess poured off, and the collodion allowed to dry for four to five minutes, after winch a second layer of collodion is poured in the same way When this has dried a few minutes the filter is ready for use. A properly made filter of this kind should give a colourless filtrate with a gold sol made by reduction with formaldehyde. Buchner funnels may likewise be used, no rubber The paper is joint being necessary in this case thoroughly wetted with distilled water and placed flat on the perforated plate Sufficient collodion is then poured on to cover the whole of the paper, and the excess poured off, leaving, however, a remnant of two or three cubic centimetres, which is carefully distributed round the edge of the paper by inclining the funnel about 45 and turning* it continuously until the collodion no longer flfljgg A second lot of collodion is poured in exactly the same way after the first one has dried to the desired extent, and the filter is ready for use when this is Parsufficiently dry. ticular attention must be paid to getting a sufficient rim of collodion round the edge of the paper, as this makes the ]omt and prevents the liquid from escaping underneath the paper An extremely convenient method of making small ultra-filters consists in the use of the seamless

ULTRA-FILTERS.

75

extraction thimbles, which can be obtained in a If apparatus for using them with variety of sizes vacuum is available, they may be impregnated dry, as described under (i) otherwise it is more convenient to impregnate them wet and use them as,

only a 4 to 5 ampere hand-regulated arc lamp, of holding a comparatively large volume of liquid, so that adsorption effects are minimized, and of being very easily ccntied. The optical part is cemented into a cylindrical metal casing, which is closed by a metal cover provided with bayonet ]omt and a central quaitz window for observation The ultracondenser is placed on the stage of the microscope so that the spigot on its lower side fits the opening the former the ordinary condenser being, of couise, removed and the plane mirror used for illumination The dimensions of the ultra - condenser do not permit the use of objec-

tives

of

shorter

length

than 6

mm

focal(or

FIG 13

I"), but, as the images are not geometrical, there is no limit to the eyepiece magnification permissible, and the highest power eyepiece available may be used.

The condenser is filled with the liquid to be examined by means of the inlet and outlet branches provided on the cover, care being taken not to leave an air bubble at the top of the liquid under the quartz cover The light from the arc lamp, which should be provided with a lens giving a nearly parallel beam, is then directed on the plane mirror, which it should fill completely and uniformly, and the condenser placed in position The light is now " centred, using the objective and a low power eyepiece, by adjusting the minor until the brightly illuminated spot is exactly in the centre of the field. The low-power eyepiece is then replaced by the highest power available; Zeiss's No. 18 compensating eyepiece, or an equivalent,is suitable.

The

DARK-GROUND CONDENSERS

79

most highly illuminated layer will be easily found by fo@ussing up and down, and, as this layer is at some distance from any boundary surface, the Brownian movement will be seen The gieat perfection large diffraction rings which appear and disappear round many particles indicate, of course, vertical

of the focal plane Coagulation of sols can be very conveniently studied with this condenser by running an electrolyte solution through one of the branches on the cover, or by adding a small amount of coagulant to the sol before it is filled into the apparatus. After use the condenser must be carefully washed with distilled water and thoroughly dned with linen free This applies equally to the metal from grease contact with the liquid parts If the ultra-condenser is not available, the presence of at least coarser ultra-microscopic particles can be detected by means of one of the numerous of this dark-ground condensers. " Typical forms " Paraboloid condenser, apparatus are the Zeiss the Reichert "Table" condenser, and the Jentzsch " " Concentric condenser, which latter, like the

movement out

ultia-condenser, is now made in this The methods of using and centring are with the different types, and are different slightly geneially adequately described in the makeis'

Jentzsch country.

pamphlets. Since they all depend on total reflection at tjie cover glass, slides of the thickness prescribed by the makers must be used in all cases Bothslides and cover glasses must be carefully cleaned in the following manner They are washed in hot dichromate-sulphunc acid mixture for five to ten minutes and then rinsed thoroughly with distilled The slides or cover glasses are then seized, water. one by one, with a spring forceps and, after draining off the bulk of the water, placed strong alcohol, in which they are kept until required. Immediately

8o

METHOD OF CLEANING

SLIDES.

before use the slide is withdrawn from the alcohol by seizing one corner with a spring forceps, and the adhering alcohol burnt off over a spirit lamp or Bunsen burner As soon as the slide has cooled it is placed on the condenser, ample cedar oil being

explained in the descriptive pamphlets. glasses may be treated in the same way, if this provided they do not crack too frequently should be the case, the alcohol may simply be to the flame a from at sufficient distance evaporated Before the cover glass is made prevent ignition air bubble, ready a large drop of sol, free from any should be placed on the centre of the slide, and the cover glass dropped gently on it immediately it has The essential point of the method described is cooled that slides and cover glasses are not touched with the with or fingers any textile material, as this renders them entirely useless for ultra-microscopic work, The layer of liquid between cover glass and slideused,as

The cover

of course, of very slight depth, and careful focussing on its central portion is necessary to observe particles moving freely Many particles will always be found, by suitable focussing, to have adhered tois,

the two glass surfaces. Electrolyte coagulation can be observed, though in a somewhat rough fashion, by placing a drop of solution on the edge of the cover glass, so that it can diffuse into the bulk of liquid The most convenient objects for becoming familiar

with the use of the apparatus are comparatively coarse systems, especially mastic or gamboge suspensions.

LITERATURE.Full information on the various types of condensers is to be found in the pamphlets issued by the makers Photometric investiga(Zeiss, Leitz and Chas. Baker). tion on Tyndall cone, connection between, size of particles and luminosity, etc by W, Mecklenburg, Kail -Zeitschr.,,

XIV,

172 (1914)

XV., 149 (1914)

XVI., 97 (1915),

CHAPTER XI.

CATAPHORESIS.

A SIMPLF apparatus, suitable lor practice and preliminary work, may oe made up from glass parts available in every laboratory in the manner illusA U-tube, about 250 mm. long, trated in Fig 14 provided with an inlet tube at the lowest point of The the Vjid, is supported in a suitable standinlet

tube is connected by a lubber tube, about 350 mm. long, to a funnel capable of holding about The tube is fitted with a screw clip or with 75 c c one of the patent clips provided with a catch, which allows it to be left fully opened, near the end of the

inlet tube.

Two electrodes are inserted in the tops of the limbs, consisting of foil rolled into a cylinder, the diameter less than that of of which should be about 2 Platinum is, of course, the bcsi material, the tube

mm

failing which, silver

electrodesliable to

become detached from the latter dunrig The electrodes are fixed to stout wires, which use. arc best mounted in a strip of ebonite, acting also as distance piece, and provided with terminals. The apparatus is charged in the following manner. The clip is opened and the sol to be examined poured into the funnel, the latter being held so that its edge is about 10 mm. above the bottom of the bend. Theliquid should just reach the

may be

even cylindrical solid carbon employed, but small pai tides are,

latter the f mines! is then lowered and again raised to the oiigmal level, to drive out any air which may have been tiappetl;

82

U-TUBE METHOD.

in the rubber tube, and the cock closed, with the sol Distilled standing just at the bottom of the U-tube water isjnow poured into the latter so as to fill the The funnel is then limbs to^about half their height

c-pjo)I

opJo]

ALJ

iLJ

Ml1

\/

raised until the level oj the liquid in it is about I below thai of the water in the limbs, and the cock opened full bore.

mm

The funnel

is

now very

slowly raised, the sol flows into the U-tube, and the level ol the water in the limbs rises

The correspondingly. funnel must be iaibed at the same rate, i e the level of the sol in the funnel should nevei,

be2

mm

more

than i 01 above the water

level in the limbs. II this is done properly,

the sol rises in the

withouttually

mixing

with

the water, and evenshows a sharp

boundary in both limbs.

If the funnel is raised too rapidly, or il the cock is not fully opened, the sol issues in a jet, FIG. 14. impinges on the upper wall of the U-bend, and rises unequally in the limbs,

without

filling

the necessary sharp boundary surface against the water While it is quite easy to till the tube properly -fter a little practice, the beginner will imd the pio-

them completely and without forming

U-TUBE METHOD.cedure

83

much facilitated by a loose plug of carefully glass wool, placed in the inlet tube at its junction with the bend. This checks and distributes the admission of the sol and prevents its issuing in a jet. Sufficient sol must, of course, be admitted to laise the water level in the limbs so far that the electrodes are covered completely. When this is the case they may be connected to the electric supply the lighting supply may be used, of course with sufficiently insulated wires or flexible leads. With a voltage of 200 the giadient in a tube of the dimensions described above is about 5V/cm., so that a very distinct shifting of the boundary is noticeable after 10 minutes. The bo determust, of course, " polarity of the electrodes " obtainmined, e g with the pole finding paper able for this purpose Rough measurements of the rate of travel may be made by marking the originalwashed cotton or,,

boundary and measuring the displacement (choosing whichever boundary is the shaiper) after a definitetime. The field strength is the voltage divided by the distance of the electrodes the latter is somewhat uncertain, but the total distance in the axis of the tube, i.e., twice the length of the straight limb, from the lower edge of the electrode, plus half the arithmetical mean of the internal and external circumference of the bend, may be taken as approximately;

correct.

An inlet tube, about 3 or 4 mm, diameter, leads into the lowest point of the latter, and is bent at light angles to me plane of the di awing, terminating at the top in a charging funnel of suitablebend.

For exact work a specially made apparatus is which incidentally avoids the use of lubber connections. A convenient form (after W. Nernst and A. Coehn) is illustrated in Fig. 15, It consists of a U-tube provided with two laige cocks at the junctions of the straight limbs with thepreferable,

62

84

U-TUBE METHOD

A scale of millimetres (not cubic centicapacity metres ') may be etched on the limbs. ^The limbs are provided with electrodes such as described above.The apparatusasisis

charged

follows.filled

The funnel

with the sol to be examined, the cocks A and A' opened, and then the cock B, until the sol just rises

above the large cocks, B, and subsequently A and A' also, are closed, and the small amount of sol in the limbs removed withpaper. are then the same height with distilled water and the electrodes placed in position. The cocksspills of filter

The limbsto

filled

A and

A' are now opened, and then B, which must be done very slowly and uni-

formly. The sol and the supernatant

water in the limbs now rise; sol is admitted until the

electrodesare

sub-

15.

merged, when they may be connected to the supply What has previously been

MICROSCOPIC METHOD.

85

said regai ding the electric gradient, of couise, applies equally to this form of apparatusIf measurements of the velocity of cataphoresis aie made, the results are usually reduced lo unit To give an example, the gradient, as stated. boundary travels to the anode, the displacement in 10 minutes The velocity amounting to 22

second is accordingly 2-2/600 cm. = 0-00366 5 With a voltage of 240 V and a 366 X io~~ distance of 24 cm between the electrodes the = 10 V/cm The velocity reduced gradient is 240/24 to unit gradient is, therefore, 366/10 x io~ 5 = 5 is a normal value for the more io~~ which X 36per

mm

highly dispersed gold sols. Measurement of CataMicroscopic Observation and " " conAny one of the dark-ground phoresis, densers described in the chapter on optical methods

A slide must be of examination may be employed provided with electrodes having parallel edges, the is contained which The elecbetween liquid trodes are two strips of metal foil platinum is, of course, preferable, failing which silver may be used The strips should be 3 mm. wide and about 35 to 40 mm. long for the standard microscope slide, i" x 3". They are fastened parallel to each other;

and at right angles to the length of the slide, so that the distance between the outside edges is equal to A $" cover the width of the cover glass to be used will be found convenient, this making the distance between the outside edges of the strips about 22 mm., and the distance between the inside edges, i e the distance between electrodes, about 16 to the slide, which has The strips are fastened 1 1 ---,---' - r --,"-i in the manner previously been

mm

'

1-

A small preparation made for insulating purposes) piece of the preparation is warmed sufficiently and drawn out into a thin filament about i mm. diameter.

akeady

describe'

< .

's

compound

(a

86slide

MICROSCOPIC METHODand the two

The strips of foil are then placed on a warm metal surface and a piece of the filament about 25 long laid on each strip, centrally as regards width, and at one end of the stiip, so as to clear The strips, leave a length of 10 to 15 as soon as the filaments of compound have softened, are picked up with a forceps, inverted and placed parallel to each other on the slide the requisite distance apart (the position of the outside edges can be previously marked with a diamond or a drawingpen) They should be dropped down into the coirect position without subsequent shifting, to avoid As soon as smearing the slide with the compound the strips are in position the slide is removed from the warm surface and placed on a clean piece of filter paper resting on a wood or glass table the strips are then weighted by placing them on a second microscope slide and on this a 5o-gm. weight from 10 to 20 minutes according to Sufficient time the temperature of the room must be allowed for the complete hardening of the cement. Electric connection to the electrodes may be made simply by pressing the wires down on the projecting ends of the electrodes, of course taking care to insulate the latter from the stage of the microscope by a piece of thin sheet rubber or the like. For repeated use it is, however, more convenient to solder leads of thin flexible cord to the ends of electiodes, the opposite ends being provided with terminals kept apart by a distance piece of ebonite. The flexible cords should be of sunicient length to allow the terminals to rest on the table, out of the way of the mirror, when the slide is in position on the stage of the microscope. The slide is placed on the stage and optical connection made with the particular condenser used in the manner prescribed for it The slide if

mm

mm

should,

possible,

be clamped down to avoid accidental

MICROSCOPIC METHOD.shifting

87

large drop

of the liquid is

then placed

centre of the space between the electrodes ; be of sufficient size to cover the field between the edges of the two strips, and make good contact with both, when the cover glass is put on. The microscope is then focussed on the central layer of liquid, so as to observe particles which are moving In view of the freely, and the current turned on short distance between the electrodes a supply at 4 to 5 V is sufficient (say, from accumulators or drythis shouldcells)

n the

The

velocity

may be measured by means

of

Release an eyepiece micrometer and stop-watch the latter when a particle under observation passes through one of the numbered divisions, and arrest the watch when it has travelled, say, through five The actual value of the micrometer divisions reading must, of course, be known, or determined The in the usual way with a stage micrometer.voltage at terminals/distance potential gradient is of electrodes in centimetres To obtain the velocity of the particles in unit gradient, i e one volt per centimetre, divide the velocity actually found by For instance, a particle is found to the gradient Five travel five divisions of the scale in 32 seconds,

the gradient is 4/1-6 To 1-6 cm 2-5 V/crn. obtain the velocity in unit gradient, the figure found above must, therefore, be divided by 2-5, so that we finally obtain the velocity per second in a field of "5 one volt per centimetre 30 x 75/2-5 X 10 IO -5 cm< xhis is a normal value for a metal sol,,

The microscopic method is particularryTconvenient and rapid for detei mining the sign of the electric charge when only small quantities of the liquid1

*"

88

MICROSCOPIC METHOD.

under examination are available When for this purpose, remember that the image is so that, if the anode is on the light, nc charged particles will travel to the left in of vision. Measurements, however, may be to fairly considerable errors unless a layci tides is observed which moves freely andthe influence of the two glass surfaces, designed to minimize the sources of ci ror has been described by Th Svedberg, a reference t will be found the literature given below.

LITERATURE.

For the microscopic method gcneially, see A and H Mouton, " Lcs ultramicroscopcs, etc For elimination of cnors, Th Svcclb (1906).Andersen, Koll.-Zettschr.,

XXIV

155 (1919)

CHAPTER XII.

ELECTROLYTE PRECIPITATION OFSUSPENSOID SOLSstarting experimental work the student should commit to memory a lew typical figures for the concentrations of uni-, bi- and tn-valent ions

BEFORE

which produce

The

pi capitation in following aie representative

suspensoid

sols

Sol.

Sign

of

Piunpitation concentration

m railhmoles/litre.A1C18 o 09 A1C18 o 01 A1C1 S o 2

AsoSn

Negative,,

PtMastic

Fe(OH) 8

Positive

NaCl 51-0 NaCl 25 NaCl 1,000 NaCl 925

CaCl2 BaCIjCaClja

o 65

006254

K SO

020

These figures give the concentration existing in 18 c.c of if, e g the mixture of sol and electrolyte the As 2S 3 sol is to be coagulated by 2 c.c of NaCl millmioles to contain have latter will the 510 solution, of NaCl, since by the addition of the sol it is diluted to one-tenth of its original concentration. A second point to be noted is the difference in the between the corresponding values, say, for NaCl, a This may be specific, i.e three negative sols to stable electrolytes given sol may be much more is the case with the such others than generally mastic suspension according to different observers The precipitation concentration, however, also this partly depends on the concentration of the sol, and,,

go

ELECTROLYTE PRECIPITATION.

Pt

explains the difference between the As 2 S 3 and the sol, the former being much more concentrated The figures for the As 2 S 3 sol may be taken as

T t} pical for this

and

sol,

as described, while the figures for the

for the (dilute) Prussian blue Pt sol will

be found approximately correct for the gold sol made by the formaldehyde method. The Prussian blue sol will be found the most convenient for theexperiments The next point to be considered is the method of adding the electrolyte solution to the sol A number of the classical investigations were carried out bytitration, \ e , by adding electrolyte solution to the sol until perceptible coagulation took place Thisfirst

method has the drawbacks that, unless there is a very marked colour change, as with red gold sols, it is by no means easy to notice an exact end-point, that it excludes the time factor, and that the concentration of sol vanes with different amounts of Nevertheless the method coagulating solution gives a rough idea, and may be used for preliminarytrials,

care being taken to use fairly large volumes

of sol

and to place the beaker containing

it

during

titration so that small changes in colour or turbidity can be readily noticed The procedure to be adopted for exact determinations is as follows a uniform volume of sol is fixed upon, to which is added a definite fraction of coagulating solution, the concentration of which ^s varied In this way the sol concentration is kept uniform The sol and solution are mixed by a uniform procedure, say closing the test tube containing it and reversing it twice or four times the mixture is allowed to stand for a definite time, say two or three hours, and is then examined. Eighteen c c of sol and 2 c.c of solution will be found convenient, the latter being diluted by the sol to one-tenth of its original concentration, Asiabout.

ELECTROLYTE PRECIPITATION.51 milliniolcs of NaCl, 0-65 of,

gi

CaCl 2 and 0-09 of A1C1 3 respectively are the concentrations required in the mixture to produce precipitation, the concentrations of the solutions used will have to be ten times 510 milhmoles of NaCl, 6-5 of CaCl 2) greater, viz and 0-9 of A1C1 3 Since these concentrations just produce precipitation in a certain sol, it will be desirable to have a consideiable maigm, and the following three stan"dards for negative sols should be prepared, . .

NaClCaCL,Aid,,*

.15

1,000 millimoles,,

= = =if

58-5 1-665

0-267

gm gm gm

in a

litre.

the materials are of doubtful purity, these solutions should be standardized against suitable standard solutions Since one part of these solutions added to nine

For very accurate work, or

{<J

\I

parts of sol will certainly produce coagulation, more dilute solutions of known strength must be pi epared Do this by mixing in test tubes, say, 8 c c of solution with 2 c c. of water, 6 c c. of solution with 4 c c. of water, 4 c c. of solution with 6 c.c of water, and 2 c c of solution with 8 c c of water ; label these being their the tubes 0-8, 0-6, 0-4 and concentrations referred to the stock respective

o%

solutions

f1

Now place 18

c c of the sol to

of five test tubes, label

them

i,

be examined in each 8, 0-6, o 4 and 0-2,

|Lr

'4

add to each 2 c.c. of the con espondmg solution, mix by the standard method decided upon, and allow the tubes to stand for a definite time, say two or that the It will then be found, c g three hours contents of i, o8 and 0-6 have been precipitated, The but that those of 0-4 and 0-2 have not changed limit concentration accoidingly lies between the,

electrolyte concentrations prevailing in 0-6

The

solutions added,

if

and 0-4 NaCl was used, contained

92

ELECTROLYTE PRECIPITATION.,

1,000 x 0-6 and 1,000 X 0-4, i e 600 and 400 millimoles respectively since they were diluted to onethe tenth by the sol, the actual concentrations The minimum mixture are 60 and 40 millimoles,

concentration necessary for precipitation lies between to determine it more accurately, an these two intermediate concentration of the added solution may now be tried, say 0-5 of the original For this purpose mix, say, 2 c c of stock solution with 2 c c of water, and add 2 c c of the mixture to 18 c c of sol as before If precipitation just occurs within the standard time, the limit concentration is obviously 1,000 x 0-5/10 == 50 millimoles per litre The procedure just described should be carried out with several sols, e g Prussian blue, arsenic trisulphide and gold reduced by formaldehyde, with all three electrolytes, and the results tabulated The results should be compared with the numerous data given in the literature and carefully checked if they show very marked deviations from the aveiage, The contents of the sols in disperse phase should also, for comparison, be calculated from the data given for their preparation similar procedure should be adopted with the feme hydroxide sol, the only representative of the positively charged sols By reference to the table at the beginning of the chapter we find that the limit concentrations are 9-25 millimoles of NaCl and 0-20 of 2 S0 4 (Na 2 S0 4 may be used instead) using the same ratio as before, 2 c c of solution to 18 c c of sol, the concentrations of the former will be 92-5 and 2-0 milhmoles, and to have the same margin as before we shall require stock solutions of the following concentrations,

NaCl

Na 2SO 4 4 The NaCl solution

.

200 milhmoles

= =

117 gm

ma

litre.

0-568 grn

may,

of course,

be made up from

ELECTROLYTE PRECIPITATIONthat previously used for negative sols with four volumes of water.

MUTUAL PRECIPITATION OF SUSPENSOID

SOLS.SOLS in which the disperse phases carry oppositechargesdefinite,

precipitateratios,

each other when mixed in while no precipitation occurs if an

is

excess of either sol

present.

sol described above will, an equal volume of generally speaking, precipitate the Prussian blue, the gold sol reduced by formaldePlace 5 c c of each mastic suspension hyde, or the of the negative sols into test tubes, add to each 5 c c. of the (dialysed ') ferric hydroxide sol, mix by a uniform procedure, and allow the tubes to stand. The Prussian blue and the gold sol will generally show coagulation within a few minutes, while the mastic suspension may take 15 to 25 minutes. The coagulum contains both disperse phases, so that the liquid in the test tubes is colourless after the former

The

ferric

hydroxide

has

settled If precipitation fails to occur with any of the sols mentioned, or with any other negative sol mixed with an equal volume of ferric hydroxide sol, the correct ratio must be ascertained by experiment. For this purpose place in test tubes i, 2, 3, etc up to 9 c.c. of ferric hydroxide sol and add (in the same order) 9, 8, 7, etc , down to i c.c. of the negative sol. The contents of each tube must be mixed, by a uniform procedure, immediately after the second sol has been added After, say, one hour note the ratio in the tube or tubes in which coagulation has occurred.,

MUTUAL PRECIPITATION OFExamine theinferric

SOLS.

95

electrical condition of

two mixtures

which no coagulation has occurred, one having

hydroxide, and the other negative sol in excess. For this purpose note the ratios and then make up a sufficient quantity of the mixtures for cataphoresis in the U-tube The sign of the charge in the mixture will be found to be that of the sol present in excess, the charge on the other sol having been reversed.

CHAPTER XIV.

PROTECTIONTHE protective effect of emulsoids may be demonstrated two ways. The emulsoid may be added to one component of a reaction which produces a precipitate and may cause the latter to become much

more highly

aqueous mediumlytes, t e,

disperse than it would be in a pure Or the emulsoid may be added to an existing sol, in which case it protects it from electro-

the concentration of the latter necessary to produce coagulation is considerably increased To demonstrate the formation of a highly disperse precipitate in the presence of a protective colloid, dissolve 0-5 gm of crystallized barium chloride in 50 c c of water, and 0-25 gm of ammonium sulphate in 50 c c of water Add 5 c c of the first solution to 5 c c of the second, and note that the bulk of the a few minutes precipitate settles Warm the ammonium sulphate solution to about it add to c c. of 15 per cent gelatin sol, and C. 5 30 mix thoroughly, and then add the barium chloride solution with continual stirring The precipitate does not settle out on standing, and the mixture a close filter through passes paper without leaving any residue Many other precipitates may be this way by adding to one obtained as sols solution varying amounts of gelatin, albumin or gum arable, and by choosing suitable concentrations The other procedure is to add to a sol, for which the electrolyte concentration required to produce coagulation in a definite time has been previously determined, small amounts of gelatin, albumin or gum arable sol and to determine what concentration

GOLD NUMBERS

97

of electrolyte will now produce a marked change or Thus, 9 c c of the gold sol made rapid coagulation by the formaldehyde method (p 30) turns blue within a few seconds after the addition of i c c of

(Watch this change by pure N/i NaCl solution transmitted light, say by looking through the test tube at a uniformly illuminated screen of white reflected light must be excluded, as the paper strong reddish-brown surface colour of the sol is almost the same for the original red as for the blue sol and makes observation of the change difficult.) Now add to 9 c c of the same sol i c c of a o-i per 100 c c ), mix well, add cent gelatin sol (o-i gm i c c of the N/i NaCl solution, and note that no Add more change of colour occurs, even on standing NaCl solution, i c.c at a time, and note that even whatever or c c no 5 4 change produces Gold Numbers The gold number of an emulsoid number of milliis denned by R Zsigmpndy as the grammes of the ernulsoid just sufficient to prevent a colour change in 10 c c of a standard red gold sol on addition of i c c of a standard solution of NaCl As (density 1-07, ^e, concentration about N/i). these figures aie arbitrary, it is better to state the,

percentage concentration of emulso^d in the gold sol which just prevents the colour change when i c c of N/i NaCl solution is added to about 10 c c of sol The following figures have been calculated from

Zsigmondy's gold numbers

MinimumEmulsoidconcentration in per cent which preventscolour change

Gelatin

0-00005 to o-oooiO-OOIto 0-002 to 0-0025 to 0-20

Egg albumin

(dry,.

com. .

Gum

mercial) arabic

Dextrin Potato starch

.

0-0015 o-io0-25

98

GOLD NUMBERS.,

These are exiguous concentrations, especially foi the most active protective agents if i c c of emulsoid sol is to be added to 10 c c of gold sol, the concentrations of the former will have to be times those given in the table, & g 0-00055 to o-oon per cent or 0-0055 to o-on gm per litre, for gelatin It is advisable to make up sols of still greater concentration, say 50 times those given in the table, preferably by suitable dilution of any concentrated sols available in the laboratory Then proceed as follows Place 10 c c of gold sol,

which should be a pure red without any purple tinge, into each of a number of test tubes, add o-i, 0-2, 0-4 up to i c c of emulsoid sol and mix Then add to each test tube i c c of N/i NaCl and note the in concentrations the tube which just retains the red colour and the next one, which shows the change toblue of If, for instance, the tube with 0-2 c.c emulsoid sol remains unaltered and the one with o i has turned, the emulsoid concentrations arerespectively

2/102 i/ioi

= 0-0196 x = 0-0099 x

original emulsoid concentration

The gold number in percentage lies between these values and may be determined more exactly by using intermediate volumes or by reducing the concentration of the emulsoid sol used and repeating theseries.

The gold numbers

tiating

are a delicate

means

of diffeien-

between proteins which cannot readily be

by other tests The method has, therefore, acquired some importance in medical and biochemical work, which is, however, beyond thedistinguished

scope of this book Behaviour of Sols, The protective effect different of a given emulsoid sol is not necessarily the same on other sols as on gold sols apart from the question;

SPECIFIC DIFFERENCES.

99

of concentration there appear to be specific differences, although, generally speaking, the various emulsoids stand in the same order for most sols as To show this, determine they do for gold sols.

approximately the volume of N/i NaCl solution necessary to coagulate 10 c c of the (dilute) Prussian blue sol Add i c c of the o-i per cent gelatin sol previously used with gold sol to 9 c c of Piussian blue sol, then add the volume of sodium chloride solution found sufficient to coagulate the unprotected sol Generally this will be sufficient to precipitate the

protected sol after a somewhat longer time, notwithstanding the presence of an amount of gelatin which, in the previous experiment, completely protected the gold sol.

LITERATUREFor protective effect and gold numbers, see ZsigmondySpear, "Colloidal Chemistry" (Chapman and Hall, in "Investigations on a Number of 1917), pp. 106,

Protective Agents, chiefly of Vegetable Origin, "by A.

only suitable instrument for accurate determinations is a properly designed capillary viscpmeter. The various rough methods employed occasionally, such as determining the time required for a given volume to flow from a pipette, or the time taken by a small sphere to fall through a given height, are time useless, as in all these arrangements the

measured is very far from being simply proportional to the viscosity Two types' of capillary viscometers may be used the simple Ostwald type (Fig 16), in which the flow is caused by the difference of head in the two limbs of the instrument, and the Ubbelohde type (Fig 17), in which the flow is caused by a constant air pressure applied to one limb. The use of the latter and the rnanostatic apparatus for providing constant air pressure will be described later. The Ostwald viscometer consists of a wide tube,generally provided with a bulb at the lower end, which is joined by a bend to a straight capillary tube The latter leads into a bulb capable of holding 2 to 3 c c. of liquid, and provided with an inlet tube, a constriction being provided where this tube joins the bulb. Marks are placed, one in the centre of the constriction and one below the bulb at the beginningrally

The instrument is geneof the straight capillary. used only for determining relative viscosities, e g., the viscosity of a sol referred to the viscosity of

all

the pure dispersion medium as unity. In this case the factors in Poisemlle's formula which depend

VISCOMETERS.

101

on the measurements of the instrument remain the same, and the pressures in the case of two liquids

FIG

17,

is

tional to these densities, provided the same volume used, so that the effective height of the liquid is If the times taken by the the same in both cases.

having the densities

Po

and

Pl are simply propor-

102

OSTWALD VISCOMETER.

level of the liquid in sinking

from the upper to the lower mark are respectively tQ and t v the viscosities the following ratio and are r]

*li

and the relative viscosity

i

j*o Po

accordingly necessary to determine the density of the liquids under examination, and it need hardly be added that this must be done at the temItis

perature or temperatures at which the viscosities are

to be

measured

To use the instrument it is first mounted vertically, by sighting it with a plumb-line in two directionsdefinite volume of at right angles to each other liquid is then run into the wide tube from a pipette convenient length of reserved for this purpose rubber tube is fitted to the top of the bulb and the until it has the drawn capillary through liquid up

filled

the upper bulb and risen well above the upper mark, when the tube is closed by pinching with the

hand (Sufficient liquid must be delivered by the pipette to leave some liquid in the lower bulb when this has been done ) A stop-watch is held the right hand, the rubber tube released and the watch started when the level of the liquid passes through the upper mark, and stopped when it passes The eye should be kept on through the lower mark the wscometer and not on the watch As visconieters are not readily obtainable, theyleft

will generally

have to be blown specially, when the For sols with following points are to be noted. water as dispersion medium and, therefore, requiring the time of outflow of water to be determined, bore will be capillaries about 0-5 to 0-6 The length of the capillary should be suitable 80 to 100 times the diameter of the bore, say 5 to

mm

OSTWALD VISCOMETER.6

103

at least bulb holding 2 to 2-5 c c on the end of the capillary is convenient, and the diameter of the inlet tube, as well as that of the bend connect-

cm

ing the capillary with the wide tube, should not be less than 3 mrn The change preferably 4 diameter from the bulb into the capillary, and from the latter into the bend, should be smooth and The time of flow should not be less than gradual 60 seconds for water at 20 C Viscometers conforming to this description will be suitable for measuring viscosities up to 20 or 25 times that of water, for fairly concentrated e,, emulsoid sols Sols having organic dispersion media, such as rubber-benzene sols, or sols of nitrocellulose in various media, however, often have viscosities of much larger order even in moderate concentrations, and it is then not feasible to carry out the whole series of measurements with the instrument used for the dispersion medium A range of viscometers of,

mm

i,

increasing bore, and proportionately increasing The length of capillary, must then be provided readings made by two instruments, say one used for the dispersion medium and for relative viscosities up to 20, and the next instrument, are then connected with each other in the following manner The timefirst

most concentrated sol, for which the instrument can be used, is determined and found to be n l The time of efflux for the same sol is then determined the second instrument and found to be a smaller value, n z To reduce readings on the second instrument to those on the first, and eventually to relative viscosities, they must, thereInstead of the fore, be multiplied with i/ 2 sol itself any sufficiently viscous liquid may be used to determine this ratio glycerine or mixtures of glycerine with (little) water may be employed. Viscometers must be thoroughly cleaned immediately before use with hot dichromate-sulphuncof efflux for the

104

THERMOSTAT

acid mixture, followed by distilled water and then by alcohol and ether, which is dried off by blowing

The viscometer, a at least two litres may be used thermometer divided into tenths of a degree, and the toluene regulator are supported in the water from a suitable stand, and some form of stirrer must be The beaker stands on asbestos-coated arranged wire gauze on a tnpod, and is heated by a small " " micro gas burner, which must, pin-hole or however, be sufficient to keep the water at the to make up the heat lost i e required temperature, by convection and radiation The water may be heated up to within a degree of the required temperature by a Bunsen burner, and the small burner sub,

wool through the instruThe latter precaution is necessary, as even small particles of dust may vitiate results seriously, For the in view of the small bore of the capillary same reason the liquids to be investigated should be filtered, or, where this is not possible, at least strained through a glass wool plug Since viscosity decreases to the extent of 3 to 5 per cent per degree of temperature in pure liquids, and at a much higher rate in emulsoid sols, it is absolutely essential that measurements should be earned out in a thermostat in which it is possible to Where keep temperatures constant within 0*1 C a proper apparatus is not available a beaker holdingair filtered through glass

ment.

The regulator is then adjusted stituted for it then so as to cut off about the required temperature , since it is somewhat troublesome to do this with complete accuracy, it is better to take readings within 0-2 or 0-3 of round numbers rather than to spend much tune in trying to set the regulator Thus, if measurements at, exactly to the latter say, five degrees' intervals are wanted, it will be quite permissible to work at 20-3, 25-1, 30-0, etc., provided the results are plotted accurately.

VISCOMETER CONSTANT PRESSURE.

The temperatureregulation

105

may

fail in

two ways

either the temperature may fall, although the regulator is fully open in that case the gas pressure or the or the gas regulator size of the burner is insufficient;

to cut off, although the tempeiature keeps " " this trouble is generally due to rising creeping of the toluene, which passes between the mercury and the glass, instead of raising the mercury This can be corrected by renewing the toluene and The the trouble is thoroughly cleaning mercury. much less frequent if a concentrated solution of calcium chloride in water is substituted for the toluene The determination of the density unless required for some further reason is tedious and can be avoided by the use of the viscometer illustrated in Fig 17, in which the pressure causing the flow is produced by compressed air instead of by the head

may

fail

of liquid itself The instrument has two bulbs of equal size ; one limb of the U connecting the bulbs is a capillary of suitable bore, while the other is a wider The liquid is drawn into the viscometer tube, through A, and a definite volume must be used, so the bulb on the that, when the level is at B the opposite limb. capillary side, it stands at C When pressure is applied on the side containing the capillary, the liquid rises into the opposite bulb, and, finally, the difference of levels is equal and opposite to that which prevailed at the beginning, so that the effect of the liquid head, and, therefore, of the

m m

density,

is

eliminated.

The pressure is generated by the simple manostat shown in Fig. i$ A Mariotte's bottle A discharges water thiough a rubber tube and a piece of glass tube turned upwards at a right angle into a second bottle B of the same size. A Tee-piece passing through theStopper in the top of the bottle is connected at C to a water pressure gauge and at D to 11 jc viscometer,

io6

VISCOMETER CONSTANT PRESSURE.

is

which

fitted

with a three-way stop-cock

The

active column of water is that between the bottom of the air tube of the Mariotte bottle and the top of

FIG

18

the bent tube discharging the water, and the gauge must, of course, have a limb of somewhat greater length than this height Before using the apparatus, water is allowed to flow into the lower bottle until the column in the

VISCOMETER CONSTANT PRESSURE

107

While this is being done gauge becomes stationary the three-way cock is turned so as to shut off the air from the viscometer and to tube compressed leave the latter open to the atmosphere Theviscometerliquid,is now filled with the required volume of which is drawn up into the left-hand limb above the bulb by suction applied at E The cock is then turned so as to shut off the viscometer from the atmosphere and leave the compressed air supply

also shut off

The stop-watch

is

now

got ready, the

compressed air admitted to the viscometer, and the watch is released as the liquid passes through the lower mark and arrested when it passes through the All measuiements in one series are, upper mark of course, carried out with the same air piessure, i e without altering the column of the manostat The time between marks is then directly proportional to,

the viscosity It is hardly necessaiy to add that this type of instrument, like the Ostwald viscometer, must be

In both cases it is essential kept in a thermostat to make sure that the liquid under examination has reached the temperature indicated by the thermothis is the case if two meter in the water bath readings taken at an mteival of, say, five minutes do not differ by more than i per cent at the outside Generally speaking, three determinations on the same specimen should always be made and the arithmetical mean taken as the final result, provided the three readings do not differ by more than I per If the readings decrease, the temperature cent. may still be rising, as already pointed out if, how,

ever, the decrease

tion,

becomes more marked on

repeti-

the viscosity of the sol is being reduced through This phenoits being forced through the capillary menon is quite common with many cmulsoid sols and very marked, e.g., with starch sols If the readings increase on icpetition (provided, of

io8

PLOTTING MEASUREMENTS.

course, that the thermostat is working properly), the capillary is becoming blocked, either by accidental contamination with dust, etc or by adsorption or In that case the instrument coagulation on its wall must be removed and thoroughly cleaned in the manner already described,

In general series of viscosity measurements will be carried out to determine, e g the change in viscosity with concentration, or concentration being conIn both cases the results stant with temperature should be plotted on sectional paper, to a fairly large scale, with the variable concentration (or temperaThe ture) as abscissa and the viscosity as ordmate latter is, of course, proportional to the product time x density, if the Ostwald viscometer is used, and to the time only with the Ubbelohde viscometer. The points found should, in general, lie very nearly on a smooth curve cusps or inflexions occur only where coagulation or the like takes place Points which fall outside a smooth hyperbolic or logarithmic cur^e to a marked extent are, therefore, suspect unless such disturbing phenomena are at all probable, and the particular reading should be carefully,. ;

repeated

To

will find

practise the use of the apparatus the beginner gum arabic sol convenient Five concen-

trations, say 5, 10, 15, 20 and 30 per cent,, should be prepared, filtered through glass wool and the viscosities determined at some convenient temperature, The sols say 2 pr 3 C above that of the room

should be prepared when required For viscositytemperature measurements a sol made from 30 gm. of gelatin in 100 c c of water is convenient, made and filtered in the usual way. Measurements should be begun with the thermostat at, say, 45 or 50 the temperature is then allowed to fall about 5, the regulator readjusted and a reading taken, and this procedure is continued until the setting temperature;

TEMPERATURE VISCOSITY CURVES

,

109

of the sol is reached The results may be plotted a as the products time X density directly, ^ e clearer insight is, however, gained by plotting relative viscosities with water at the same temperature taken as unity The viscosity of water at different temperatures may be found in tables, or be determined in a second viscometer placed the thermostat the densities of water at different temperatures are given in most works of reference The quantities to be plotted will then be, if the times of efflux and the densities of the sol at the temperatures are and p 1( t^ and p z> etc and the i, 2, etc corresponding figures for water t\ and p'j, t\ and p' z;

S.

curve plotted with these relative viscosities as ordmates against temperatures as abscissae shows that the temperature coefficient of the sol is much the percentage greater than that of water, i e decrease with rising temperature is much greater water than that of pure,

LITERATURE.See, generally, Faraday Soc. Gen. Discussion on Colloids and their Viscosity, 1913 copious references to experimental work are given in this, especially in Wo. Ostwald's;

contribution

CHAPTER XVI.

ADSORPTION (QUALITATIVEEXPERIMENTS).of solutes from solution by solids having a large surface can be shown in a great variety

THE removal

Solutions of dyes, e g of ways crystal violet, methyl violet, methyl green, etc containing 2 to 3 mg in 100 c c., may be shaken with 2 to 3 gin. of charcoal, fuller's earth or china clay, and will generally be found colourless after the adsorbent The adsorption of lead salts is another has settled Add to 100 c c of water 2 or 3 striking example,

drops of concentrated solution of lead nitrate, take of the mixture and note the reaction with ammonium sulphide Then shake the bulk with 4 to 5 gm of charcoal, filter and test the filtrate with ammonium sulphide with moderately good brands of charcoal no reaction, or at most a very faint brown tinge, will be visible, Dissolve 2 to Influence of Solvent on Adsorption 3 mg. of methyl violet in 100 c c. of water, shake with 2 gm. of charcoal and allow the latter to the supernatant liquid is generally coloursettle Pour it off as far as possible, and pour on the less charcoal So to 90 c.c. of alcohol or acetone. This immediately assumes a violet colour, showing that the equihbrmm concentration in the organic solvent is higher than in water, i.e the amount^adsorbed5 c c,,

ELECTRIC & SELECTIVE ADSORPTION in

60 cm, long, is held vertically in a suitable clamp. The lower end is closed by a rubber stopper, throiigh which passes a short piece of glass tube about 4 mm. diameter, the upper end of which is flush with the

Place a loose plug of glass wool on the stopper, and then fill the tube to half its height with silver sand, which has been washed with nitric acid followed by water, and has then been dried and Then fill the tube with ferric hydroxide sol ignited and collect the liquid which escapes from the outlet

surface of i or 2 cm

the

stopper,

while the lower projects

tube in a beaker This is quite colourless, the ferric hydroxide, which is positive, having been discharged and retained by the negatively charged quartz If Night blue, a dye which is also positively grains charged in aqueous dispersion, can be obtained, the sol may be used instead of ferric hydroxide 2 to 4 mg in 100 c c is a suitable concentration Similar results may be obtained by allowing strips of filter paper (which also takes a negative charge in water) to dip into sols. If the latter arc positive, only water uses, the disperse phase being coagulated at the level of the liquid if the sol is negative, no of Prussian separation occurs and the colour, e g the strip. blue, rises; ;

Selective

Adsorption

An example

can be demon-

strated as follows Dissolve 5 gm. of gelatin in 50 c.c of water in the usual manner, and pour the sol into shallow moulds, so as to obtain strips or Remove these from the discs 3 to 4 mm. thick. moulds after 12 hours, and place them in a flat dish containing about 150 c.c. porcelain (developing) of a 2 per cent, solution of commercial aluminium Place 10 c.c. of the solution in a test tube, sulphateof ammonium thiocyanate, note that the solution shows a marked iron reaction, and After lying in the solution set the sample aside. for three or four days the gelatin shows a marked

add a few drops

H2

ADSORPTION,

reddish-brown tinge due to ferric iron if a 10 c c sample of the solution, in which the gelatin is lying, is again tested with thiocyanate and compared with the original sample, the iron content will be found to be much reduced

CHAPTER XVII

CAPILLARY ANALYSIS.of separating and detecting various constituents of a mixture by means of theis

THIS

a method

difference in their rates of diffusion and adsorption The usual procedure is to allow the solution contaming the several solutes to rise in strips of whitefilter paper the various constituents rise to different heights, and may be detected by their colour, or by suitable reagents applied to different portions of,

the stripstrips should be cut from a white, neutral paper (Whatman No 2 is suitable) about i cm wide and 25 to 30 cm long the edge of the sheet must not be used. They aie then suspended with their lower ends vertically, dipping about 2 cm. into the liquid to be examined. Evaporation must be prevented for single experiments the simplest way is to place the liquid in the bottom of a tall cylinder and suspend the strip from the stopper, care being taken that it hangs vertically and does not The liquid is allowed to rise until touch the wall it becomes stationary or for a fixed time, say 6 to 9 hours, and the strip is then examined andfilter, ,

The

tested.

The following is a convenient example for showing the delicacy of the method Slice about 100 gm, of boiled beetroot, pulp the slices with 50 c c of 5 per cent acetic acid, place the pulp into a muslin bag and expiess about 50 c c. of liquid, which neednot befiltcied.

Take

5 c.c. of the liquid

and add to

ii 4it

CAPILLARY ANALYSIS.;

tube N/25 caustic soda solution gradually in a test the colour changes to purple, brown and, finally, to Now add to the 45 c c of a dirty greenish yellow of the methyl liquid three to five (burette) drops colour orange used as indicator This turns red, the of the soluthe red masked deep by being entirely the colour change with alkali is similarly tion masked, as the beetroot pigment also turns yellow. Place the liquid to which methyl oiange has been added into a tall cylinder and suspend a strip of The stnp is filter paper as previously described. gradually stained a fairly uniform purplish red of a about 16 or 18 cm. reached this has When height remove the strip, let it dram for a few minutes and then dip it into N/25 solution of NaOH, removing it immediately The clear yellow of the methyl orange turned by alkali is very plainly visible at the top, over a width of 2 or 3 cm while the rest of the strip If the strip is immestill remains purple or red diately rinsed, dried and kept in the dark, the result be preserved permanently failing this, the may lower portion gradually gets discoloured Another mixture suitable for demonstrating the method is made by extracting 5 gm of turmeric with 30 to 40 c.c of hot water, filtering and adding to the nitrate about i c c. of concentrated picric acid. The mixture stains the filter paper a fairly uniform when dipped in dilute caustic soda, the yellow lower portion turns brown, while the upper, which contains the acid only, remains yellow with sodium,

picrate.It

The method

is

capable of very wide application

its possibilities

has been developed, and

demon-

strated, chiefly

by F

Goppelsroeder, whose work

unfortunately has appeared chiefly publications not generaPy accessible A number of papers covering a very wide field were published in theKolloid-Zeitscknft,

Goppelsroeder, Kott -ZeitscHr

A COMPARATIVELY simple and satisfactory instance

is the adsorption- of oxalic acid by charcoal, the acid concentration being determined by titration with

potassium permanganateof crystallized oxalic acid Dissolve 10-5 gm Then (C 2 2 4 2H 2 0) to make 250 c c of solution into conical beakers or Erlenmeyer flasks five place of 100 to 150 c c capacity the following 50 c c of the original acid solution 40 c c of solution and 10 c c. of water , 30 c c of solution and 20 c c of water 20 c.c of solution and 30 c c of water, and 10 c c of solution and 40 c c of water Label the beakers in the same order, 5, 4, 3, 2 and i respecthese being the ratios of their original contively, centrations Place in each beaker i gm of finely powdered charcoal and shake well at intervals, and finally allow the powder to settle overnight. have now to consider the choice of a suitable The solustrength for the permanganate solution tion (i) has a concentration before adsorption one-

We

fifth of

that of the initial solution, and this will preThe final concentration after adsorption should, however, still be of accurate determination, and we shall, capable therefore, do well to choose a permanganate solution so dilute that, say, 25 c.c of it will be required to oxidize 5 c c of the starting solution.

sumably be considerably reduced

ADSORPTION ISOTHERM.

117

of the latter (C 2 2 4 2 126) is M/3, and we shall, therefore, require 4 /i5 for complete oxidation of one litre of 21-086 Since, however, we have solution, i e settled that 5 c c of permanganate solution should

The concentration*

2H

2KMn0

gm

equal I c c of the original acid solution, we shall require one-fifth of that concentration, i e 4 217 It will not be necessary to make up a per litre,

gm

litre,

but 500 c

c.

(containing 2 108

gm

should be

This leaves an with freshly distilled water for repeating any of the experiments comes to the be plotted, isotherm which, when a reserve of oxalic acid is also appear doubtful provided for by the figures given above The first operation is, of course, to determine the actual ratio of permanganate solution to oxalic acid. Titrate 5 c c of the stock solution with the permanganate solution in the usual way, hot in presence of Suppose the figure found is 26 i c c. sulphuric acid Then 5 c c of the solutions (4), (3), (instead of 25) will and respectively require 20 88, 15 66, (i) (2) 10 44 and 5 22 c c of permanganate Five c c of each of the solutions is now pipetted off, without disturbing the sediment of charcoal, and titrated Begin with (5), remember the original bear in mind that the concentration will titre, 26 i c c be perceptibly reduced and, therefore, go slowly If, as will quite possibly happen in one case or another, the end point is overrun, the determination must, of course, be repeated Assuming that the sample (5) after adsorption requires 17-2 c c only, acid equivalent to 26-1 17-2 8-9 c c has disappeared from In proceeding to the other solution by adsorption samples, remember that the amounts which have been adsorbed will be smaller absolutely but greater As charcoal vanes very considerably, relatively

made up

ample reserve

* In this chapter "M" is used to denote a concentration of one mole (gramme-molecule) per litre.

n8no

ADSORPTION ISOTHERM

The definite figures can be given for guidance. results of a series of determinations, made exactly asdescribed, are, however, given below Five c c of original acid solution requires 26 5 c c of 4

KMn0

(5)

(4)

(3)

(2)

(i)

5 c c of solution requires c c of 4 before ad-

KMnO.

sorption 5 c c of solution requires cc of KMnO,t after adsorption

,

.265.

2i-3

159

10 6

53

17-5

12 3

788-1

3-8

084-5

Difference, t e amount adsorbed, in c,c of KMnCXt

.

90

87,_

68

Since all our units are arbitrary we can write the usual adsorption formula in the simple form

n aC~

where y is the amount adsorbed and C the equilibrium concentration The latter, expressed in cubic centimetres of permanganate solution, is given bythe figures in the second row, while the figures in the We can, third row give the y in the same units therefore, plot the C as abscissae and the y as ordmates on sectional paper to a convenient scale, say i c.c, The points so obtained should he on a smooth i cm curve of the familiar parabolic type (Fig. 19), If any points fail to do so, the corresponding determination should be immediately and carefully

repeated

Although the curve obtained have the general appearance

isotherm,it is

may

be smooth and

of the adsorption not possible to say definitely that it conforms to the equation without a further test. If we take the logarithms on both sides, we find

log y

i/n log

H- log a,

which, taking log y and log C as co-ordinates, is the equation of a straight line. To test the nature of the curve we must, therefore, plot the logarithms of

y~~ C

DIAGRAM.,

119

this y and C as ordinates and abscissae respectively can be done by plotting the actual figure&on logarithmically ruled paper, or,if

this is not available,

by taking

the logarithms and plotting them to a convenient I cm on ordinary millimetre paper. scale, say o-i The logarithms in that case should be taken to three

QFIG 19

10

IS

figures,

with the last figure corrected We thus obtain the following values for the results found

above(5) (4)

(3)

(2)

(I)

log C log y

. .

1-243

1-097'94

0-954

o 892 o 908

0-580 0-833

o 903-1 o 653

= -o 097

These values have been plotted, log C as abscissae and log y as ordinates, in Fig 20, and lie very nearly on a straight line. The deviation is not greater than appears in most of the log y log C curves to be found in the literature. Whether it is due to experimental error or actually denotes a departuie from

120

LOG

LOG

DIAGRAM.

the ideal type of isotherm can only be determined by further test, i e by repetition of the titrations,

and by determining two further points, Suitable say one intermediate and one below (i) mixtures would be 15 c c of the original acid solution with 35 c c of water, and 5 c.c of the original acid solution with 45 c c of water As the acid in theof(2)

and

(i),

latter will

be almost completely removed,

it

may

be

05

05

I0

logCFIG 20

advisable to carry out the titration with 10 or 15 (instead of 5) c c the result being reduced to 5 c c by calculation. In view of the smoothness of the y C curve, the deviation from the straight line, and, therefore, from the ideal isotherm, is probably,(

real.

log y log C diagram may be used for determining the value of n in the equation of the isotherm, since n log C/(log y log a] is the cotangent of the angle made by the straight line with the C-axis

The

COMPARISON OF ISOTHERMS.

121

Calculated from the straight line joining the points this in the present case would (2), (3), (4) and (5), be 5 5, so that i/n 0-18, which, although low, comes well within the range of observed values of a of course, the value assumed by log y Log is, i/n when log C o, i e the length cut off by the straight the diagram given log a is line on the y-axis about o 68. the Although adsorption isotherm and the log y log C curve may be plotted with arbitrary units as co-ordinates, for any given solute, comparison with another substance ^s only possible if the results are

therefore know that I gm of the charcoal used, placed in 50 c c of M/5 solution of oxalic acid, leaves an equilibrium concentration of M/io 2 This enables us to compare oxalic acid with, say, another organicacid, using, of course, the same quantities a of the same charcoal trations, i e of M/5 solution of the other acid.,

We

gm

and concenand 50 c c

The example discussed has been chosen as being the solute can particularly simple for two reasons be used in fairly high concentrations, and the method of titration is a very accurate one. Similar conditions, if not quite so favourable, obtain with other organic acids, the concentrations being determined by ordinary acidimetric methods Adsorption from mixtures can be studied when a specific method is,

122

SOURCES OF ERROR.

available for titrating one constituent ; thus adsorption from mixtures of oxalic and some other acid can be investigated by determining the whole acid contentacidimetrically, and the concentration of oxahc acid, in a parallel sample, by permanganate In most cases the difficulties are, however, considerably greater, and resolve themselves chiefly into finding analytical methods of sufficient delicacy to determine small differences of small concentrations.

Thus with many dyes the whole range

tions investigated while no specific

may be muchmethod

of concentrabelow o-i per cent., of titration is available

Determinations of this kind have been made by colonmetnc methods. If the solute is optically active, concentrations may be determined by the polanscope, provided, of course, that the specific rotation does not vary with the concentration, a point which must be ascertained by experiment with a few solutions of known strength and approximately coveringthe range to be investigated.

In determining the adsorption curve the assumpmade that an equilibrium has been reached Although this is, roughly speaking, true in many cases, numerous instances are known in which small amounts of solute continue to disappear from the solution The effect of this continued sorption may show itself even in the tune which necessarily elapses between the first and the last titration, i.e the values found for the samples last examined are somewhat higher relatively than those for the first, a discrepancy which would show itself particularly in If there is reason to suspect the log y log C curve this phenomenon, the liquid should be left on the adsorbent and determinations repeated at intervalstion is,

of

some days. The causes may be various, e.g the adsorption on the coarse external surface of the adsorbent is followed by slow diffusion into the pores with further adsorption on the surface of the latter, ;

CHOICE OF ADSORBENTor chemical action

123

may follow adsorption, a possibility which, although apparently remote, has been or, finally, the proved real in some instances physical condition of the adsorbent, and, therefore,;

chlonde on charcoal a good example very low concentrations.

,

(1918), adsorption of sodium auriof adsorption

CHAPTER

XIXas

THE LIESEGANG PHENOMENON. R E LIESEGANG' s original prescription

is

follows 4 gm of gelatin is dispersed in 100 c c of water in the usual way, and 2 c c of a concentrated solution of potassium dichromate added to the sol The mixture is poured 011 clean glass plates to form a thin layer, about o 45 c c per square inch of suiface The plate is supported on a horibeing allowed 10 to 15 zontal surface and the sol allowed to set minutes will be required, according to the temperature of the room A large drop of 20 to 30 per cent;

solution of silver nitrate is placed in the centre of the plate, preferably by allowing five successive drops of about o i c c each to fall on the same spot from a small pipette drawn into a sufficiently fine point.If this operation is properly carried out, the drop The plate is should have a clean circular outline kept in the dark for 24 to 48 hours (as light acts on gelatin containing dichromate), but maybe examined from time to time in diffuse light. At the end of this period any traces of the original drop still remaining may be removed with a pointed strip of filter paper, and the gel is then allowed to diy The silver chromate resulting from the reaction will be found to form numerous concentric circles round the edge of the original drop, separated by clear zones free from precipitate and increasing in width from the centre outwards The following details should be noted The plates must be quite clean and, particular, free from To cover them with gelatin right traces of grease

SILVER CHROMATE RINGS

to the edgeis

125

an operation requiring considerable up practice, and the beginner may be satisfied with a unifoim layer extending to within I" of it. The plate should be slightly warmed and held in the left hand an inch or two above the horizontal surface on which the plate is eventually allowed to cool, while the whole amount of sol is poured slowly on thecentre and uniformly spread by slightly inclining the After cooling and before plate as may be necessary putting on the silver nitrate the plate should be placedit can be left undisturbed for the rest of the time, as the drop easily spreads if the plate is moved. To produce really good rings the gelatin must contain a small amount of acid and of gelatose (a

where

happen

hard gelatins may proportion, while particularly require a slight addition of either or both Liesegang recommends citric acid as particularly suitable, and the addition of 5 to 10 drops of a 5 per cent solution to 100 c c of sol may be tried if a particular brand of gelatin does not give good rings Similar quantities1

product of hydrolysis which does not gelatinize on Infenor commercial brands of gelatin cooling). to contain these two constituents in the right " "

may also produce marked improvement be prepared by prolonged boiling of a 10 per cent, gelatin sol (evaporated water being replaced), which is continued until a sample placed on a cold Suitable proglass surface no longer sets to a jelly. portions of either or both constituents increase the width of the chromate nngs until, with excessive amounts, the whole precipitate forms a continuous band Instead of adding gelatose it may also beof gelatose;

it

may

produced in the sol itself by keeping it at high temthe dichromate must, perature for several hours of course, not be added until this operation is complete, as it would undergo partial reduction. The experiment may also be carried out in a somewhat different manner A test tube is filled to about,

i 26

VARIOUS REACTIONS

two-thirds of its height with the dichromate-gelatin which is allowed to set, and a few cubic centimetres of the silver nitrate solution is then poured on Other reactions, however, give better top of the gel results with this procedure, among which the followare particularly suitable for study ing Dissolve 3 gm. Tncalcium Phosphate in Gelatin of crystallized tnbasic sodium phosphate (Na 3 PO 4 and pour the water iaH 2 0) in 100 c c. of distilled Allow the latter to solution on 10 gm. of gelatin swell for three to four hours, then disperse on the water bath at 100 C and filter at 80 to 90. Even the best brands of gelatin give a pi capitate with the it phosphate, but the procedure prescribed makes coarser than it would be if the sodium salt weresol,.

sol Test tubes, f or f diameter, are with the filtered sol to about two-thirds and allowed to cool slowly, The sol must be poured slowly down the side of the test tube, to avoid the forAfter the tubes have mation of froth or bubbles stood for at least one hour, any of the following solutions may be poured on, all of which give

added to the

"

"

filled

10 per cent. numerous excellent stratifications CaCl 2 a mixture of two parts of 10 per cent. CaCl 2 and three parts of 10 per cent. NaCl solution 20 per cent, crystallized calcium nitrate (Ca(N0 3 2 ^H 2 O) Formation of strata continues down to the bottom of the tube and is complete in seven to ten days. Lead Iodide in A gar Dissolve 4 gm. of potassium iodide in 100 c c of i per cent, agar sol, prepared and Pour the sol into test filtered as described above,

tubes exactly as explained in the preceding para-

graph and allow them to cool slowly. When they have reached the room temperature, pour on a 30 per cent solution of crystallized lead nitratereaction proceeds rather rapidly, fine, strata will generally be visible in the course of one hour.

(Pb(N0 3 and the

2 ).

The

first,

very

IN TEST TUBES.

127

Lead Chromate in Agar. For this experiment the the following agar has to be carefully purified manner Place i gm. of shred agar in a weighed 200-c c. beaker and soak it three changes of distilled water, allowing eight hours or thereabouts for each change After the last lot has been poured off, the total weight of water (a good deal has been imbibed by the agar) is made up to 100 gm (% e., the total weight to 101 gm. + weight of beaker), the agar is dispersed on the boiling water bath and o i gm. of crystallized lead acetate (PbA' a sH 2 0) dissolved in it The sol is strained through a plug of glass wool and filled into test tubes as before. After cooling, a solution of 0-5 gm of potassium dichromate in 100 c c. of water is poured on The stratifications, owing to the low concentrations, are

very delicate, but exceedingly numerous and regular. They form throughout the length of the test tube and, as the dichromate is in large excess, the gel is coloured a faint yellow. As agar does not adhere to glass, trouble is occasionally caused by the aqueous solution creeping between the glass and the gel. This may be prevented in the following manner a sol containing f L 10 grn. of gelatin and 3 fT~ r T>-T r.-~ dichromate in 100 c.c. of water is ; ;! -.est tubes to j be used are filled with the sol, emptied with constant turning round their axis, so that a uniform coating of gelatin is left, and allowed to cool with their open ends downwards. They are then exposed to direct sunlight or full dayhght for several hours, duung which time the gelatin coating dries and becomes insoluble. Finally they are filled with water, which is changed until it remains quite colourless, emptied and dried. Agar adheres perfectly to the tanned gelatin surface obtained in this fashion. Many other reactions may be studied in either gelatin or agar gels, particulars of which will be:'

'i

128

MOLAR CONCENTRATION.

found in the literature The following points should If the aqueous solution generally be rememberedto diffuse into the gel at all, its molecular concentration must be in excess (generally considerable) of that in the gel The concentration need not, however, be exclusively due to the reacting solute, but may be partly made up by an inert salt Thus in the tricalcmm phosphate reaction, solutions of calcium chloride alone, or mixtures of calcium and sodium chloride, give good results in the latter the concentration of CaCl 2 is lower, but the total molar concentration, CaCl 2 NaCl, is as high, or higher, as with CaCl 2 alone This particular expedient always deserves trial when solutions at reasonable concentrations do not give good results, or when salts of low solubility have to be tried, which do not diffuse into the gel with sufficient rapidity at the highest attainable concentrations Differences in the qualities of the gelatin used, and in the procedure adopted in prepanng the gels, may affect the results profoundly this is particularly the case when the salt dissolved in the gelatin is not neutral in the concentrations employed, eg, Na 3 P0 4 Agar is less variable and is also much less affected by many substances which attack gelatin, such as acid or alkali liberated by hydrolysis, so that it is to be preferredis,

when possible A reaction which gives good results in gelatin, however, generally does not do so in agar, and vice versfi ; thus Liesegang's reaction does not lead to good stratifications in agar, while the lead iodide reaction does not produce them in gelatin It is nevertheless sometimes possible to obtain stratifications with a combination that does not produce them directly, by the intermediate formation of a reaction product which appears that form, Two examples may be tried as follows Liesegang's Silver Chloride R^ngs Disperse 2 gm, of gelatin in 20 c c of water and add i c,c of 20 per

m.

SECONDARY STRATIFICATIONS.cent, solution of

129

AgN0 8

Cover a glass plate, about

5"

Xit,

7",

with the

on

in the

sol and allow it to set. Then place manner described for the silver chi ornate

experiment, a large drop of 20 per cent solution of NaCl. The latter diffuses into the gel and forms If, AgCl, which is deposited as a continuous zone. however, a few small grams of silver chromate e the precipitate of varying composition obtained (i by mixing solutions of dichromate and silver nitrate) are placed at points about 10 to 15 mm. from the edge of the original drop, nngs of silver chloride are formed beyond them as the NaCl diffuses to that This is, of course, due to the formation of distance sodium chromate and dichromate, which diffuses into the gel containing AgN0 3 with the formation of the usual rings, which, however, are tiansformed into AgCl when the solution of NaCl reaches them. Lead Chromate in Gelatin, Begin the Liesegang experiment exactly as described. When a ring of wide has formed, remove silver chromate 2 or 3 the drop of silver nitrate completely with blotting and replace it by a drop without it, spreading paper, The lead of 30 per cent, lead nitrate solution replaces the silver in the chromate formed, while the fresh diffuses ahead, rings, forming resulting AgN0 3 Lead nitrate placed directly on the dichromate etc. gelatin does not form nngs, but only a continuous band. Two methods are Reactions in Silicic Acid Gel A silicic acid sol is prepared in the manner possible. described above, and the one reaction component the dissolved in it to the required concentration sol is then filled into test tubes and allowed to set. This method is attended with several difficulties. Some salts, e g., iodides or thiocyanates, retard the Repeated heating of the setting very consideiably. sol, but not to boiling, may reduce the time required Other salts, such as carbonates for coagulation. and phosphates, promote setting to such an extent,,

mm

I3 o

REACTIONS IN SILICIC ACID

that they frequently cannot be dissolved completely before it occurs The alternative method is to decompose the sodium silicate with the acid of which it is desired to form insoluble salts, and to use directly the gel thus obtained, which, of course, contains the sodium salt 10 to 15 per cent solution of of the acid used crystallized sodium silicate is a suitable starting material (prepared with boiled distilled water and dilute acid is then prefiltered, if necessary). pared, containing in a given volume approximately the amount necessary to decompose the sodium silicate, calculated as Na 4Si0 4 contained in the same volume of silicate solution preliminary trial is then made by adding to a known volume of the dilute acid some methyl orange and titrating with the sodium silicate solution until the mixture is ]ust It is then set aside and allowed to coaguneutral if coagulation occurs within a reasonable time, late say 12 hours, the ratio of acid to silicate may be adopted. The necessary quantities of dilute acid and sodium silicate are thoroughly mixed (of course without the addition of any indicator), and the mixture poured into test tubes and allowed to set, when the aqueous solution is poured on. The gel obtained by decomfollowing give fine results position with HC1 and, therefore, containing NaCl, of cent solution Pb on it 25 per (NO 3) 2 gel obtained by decomposition with phosphoric acid and containing sodium phosphates, on it 20 to 30 per cent,

solutions of

CuS0 4 BaCl 2 Sr(N0 3

,

2,

MnCl 2

etc.

Manyselves.

other combinations will readily suggest them-

by

The plates obtained Preservation of Specimens. Liesegang's method are allowed to dry and may If exposed then be kept indefinitely in a dry place to the atmosphere the silver chromate is, however, superficially transformed into sulphide, which shows

PRESERVATION OF SPECIMENS,

131

the colours of thin films. This may be prevented by a cover plate cemented on with Canada balsam. The specimens in test tubes must be kept from drying and, in the case of gelatin, from putrefaction. It is advisable to harden the latter with a 2 per cent,

The aqueous solution formaldehyde is pouied off and replaced by the formaldehyde solution, which is allowed to diffuse into the gel for three or four days and is then poured off The tubes, as well as those containing agar specimens, may then either be drawn out and sealed, or closed with corks covered with paraffin or sealing wax If the tubes are sealed off, this must be done very slowly so as not to form a vacuum in the upper half of the tube, since this causes the formathe gel, which disfigure the tion of gas bubblessolutionofleft in